In the text below, I look at some of the most important features of vernacular Finnish as it is spoken in Jyväskylä (heretofore referred to as VFJ). My claims are not based on careful linguistic studies, but on personal observations I have made during the 25 years I have lived in Jyväskylä. So, take anything you read below with a grain of salt. Nonetheless, many of my observations below also apply to the vernacular variants of Finnish spoken elsewhere in the country.

Demonstrative Pronouns

In VFJ, the following demonstrative pronouns are most often used:
Singular: tää, tuo, se
Plural: nää, nuo, ne

For tuo and nuo, the forms toi and noi are sometimes used, but these are more common in the Tampere area and in southern Finland. The demonstrative pronouns se and ne are also used as third person personal pronouns; see the next section for details. For tuo, the stem to- is used for all cases other than the nominative.
Consult the following table for the irregularities in declination:
 

As you can see, two different forms occur in the genitive plural. I find myself using either rather arbitrarily, and I believe this is quite common in VFJ. The final /n/ of the genitive is weak; see the section on possession for details. Notice also the weak final /i/ of the translative case.
 

Personal Pronouns

In VFJ, the following personal pronouns are most often used:
Singular: mää, sää, se
Plural: me, te, ne

The length of the vowel in mää and sää can vary. It is usually quite distinctly long when the pronoun is stressed. When it is unstressed, the length varies between short and half-long depending on the speaker. The 3rd person pronouns correspond to the demonstrative pronouns, and are declined similarly.
Unlike in some other vernacular variants of Finnish, the pronouns hän and he of the standard language are extremely rare. In my ears, they sound terribly pretentious when used in casual conversation. Generally speaking, the younger the person, the less likely the use of hän and he. On the other, my parents, for example, have probably never used them in casual situations. Their use is extremely rare even in semi-formal situations, such as talking to a complete stranger on the telephone.
The stems used for cases other than the nominative are the following:
Singular: mu-, su-, se-
Plural: mei-, tei-, nii-
The inflection of the personal pronouns is somewhat irregular. All the forms can be determined by consulting the following table:
 

The following examples show what I believe to be the most common expressions of possession:
 

When the following noun begins with a vowel, the final /n/ of the genitive case quite often becomes a long glottal stop [?:]; e.g. [mu?:auto]. This does not happen with all speakers, and even when it does, the /n/ may still occur in more careful speech. When the noun begins with a consonant, the /n/ is almost always preserved, although it assimiliates to the following vowel; e.g., mun kirja [muNkirja].
Possessive suffixes occur rather rarely, the -s of the second person singular being the most common of them.
For the 3rd person singular and plural, the possessive suffixes -nsa/nsä and -sa/sä are used, usually only without the 3rd person personal pronouns. I cannot say whether the n-less form is less or more common -- I use both forms quite arbitrarily. When it does occur, the 2nd person plural possessive suffix, -nne, is also most commonly used without the corresponding personal pronoun.
For example: Ne vei ne auto(n)sa pois. 'They took them cars of theirs away.' Käske sen tulla hakemaan auto(n)sa pois. 'Tell him/her to come and take his/her car(s) away.' Ottakaa ne suksenne ja painukaa helevettii! 'Take them skis of yours and get the hell outta here!'
The 1st person possessive suffixes -ni and -mme are much less common (especially -mme), but are occasionally used with a noun in the accusative or genitive case. When they are used, they are most often unaccompanied by the corresponding personal pronoun. I personally use -mme and -ni in writing only.
 

Verbs

Some verbs are inflected quite differently in VFJ. One of the most remarkable things is that the passive form is commonly used in first person plural. In fact, this is quite a common phenomenon in vernacular Finnish. So, instead of saying me menemme, 'we go', we say me mennään. The same applies to all the tenses: me mentiin (past), me ollaan menty (perfect), me oltiin menty (past perfect). Notice also the irregular conditional mood: me mentäis pro me menisimme (present), me oltais menty pro me olisimme menneet (past). The potential mood, of course, is extremely rare in vernacular Finnish.

In VFJ, some common verbs are inflected somewhat irregularly in the indicative and imperative present tense:
 

Literary Finnish	minä olen	sinä olet	hän on	me olemme	te olette	he ovat
VFJ	mää oon	sää oot (oo!)	se on	me ollaan	te ootte	ne on

Literary Finnish	minä menen	sinä menet	hän menee	me menemme	te menette	he menevät
VFJ	mää meen	sää meet (mee!)	se menee	me mennään	te meette	ne menee

Literary Finnish	minä tulen	sinä tulet	hän tulee	me tulemme	te tulette	he tulevat
VFJ	mää tuun	sää tuut (tuu!)	se tulee	me tullaan	te tuutte	ne tulee

The regular inflection in VFJ is given in the following table:
 

Literary Finnish	minä istun	sinä istut	hän istuu	me istumme	te istutte	he istuvat
VFJ	mää istun	sää istut	se istuu	me istutaan	te istutte	ne istuu

Notice that the third person plural form is exactly the same as the third person singular.
Verbs are mainly inflected normally in the past tense, except for the first person plural (e.g. me mentiin) and the third person plural (e.g. ne meni).
The final /i/ in the 3rd person singular of verbs in the conditional mood is very weak; e.g. hän istuisi > se istuis, hän söisi > se söis.
In the perfect tense, the finite component olla follows the vernacular inflection: mää oon menny, sää oot menny, se on menny, me ollaan menty, te ootte menny, ne on menny. If in these examples the active past participle of mennä seems odd to you, you're right: the final consonant /t/ of the active past participle has become the glottal stop, which causes gemination of a following consonant. E.g. se on menny kotiin [seommennykkotiin] pro hän on mennyt kotiin.
The auxiliar älä, älkää used for the negative imperative has the vernacular variants elä, elkää. E.g., älä mene sinne! > elä mee sinne! In fact, the initial vowel may vary quite a bit while remaining somewhere in the vicinity of /e/, /E/ and /&/. I my own pronunciation, the initial vowel is probably closest to /E/ most of the time.
 

The Interrogative Particle -ko, -kö

The interrogative particle is quite weak in VFJ when attached to verbs.
In the second person singular, it disappears completely, so that the prounoun immediately follows the person suffix of the verb. E.g. ootsää pro oletko sinä, meetsää pro menetkö sinä, tuutsää pro tuletko sinä, syötsää pro syötkö sinä, etc.
In the other persons, the particle becomes /ks/. E.g. oonksmää pro olenko minä, onkse pro onko se/hän, mennäänksme pro menemmekö me, kävittekste pro kävittekö te, tietääksne pro tietävätkö ne/he, etc.
 

Numerals

Various forms of the cardinal numerals occur in VFJ. In the following summary, the shortest forms are used for fast counting (i.e. yy kaa koo nee vii etc.)
The suffix -toista occasionally loses its final vowel. It can also be pronounced as -toi.
 

• A /d/ between vowels sometimes disappears. When this happens, some assimilation may happen with the vowels preceding the /d/. For example: myyä pro myydä, syyä pro syödä, juua pro juoda, viiä pro viedä, etc.
• Vowel combinations in unstressed syllables occasionally lose the second component while the first component becomes long. E.g., vaikee pro vaikea, vihree pro vihreä, kylpee pro kylpeä, etc. When such vowel combination comprises the partitive suffix, this rule does not usually apply. E.g. seiniä, whereas seinii is uncommon in VFJ.
• Diphthongs with a final /i/, when occurring in unstressed syllables, tend to lost the /i/ component to become monophthongs. E.g., keltanen pro keltainen, valkonen pro valkoinen, se istu pro hän/se istui, etc. This does not happen when the /i/ denotes plural, as in autoissa. When the /i/ indicates past tense in verb inflection, this loss of /i/ does not usually occur, except when it occurs in word-final position. E.g. mää ajoin, but se ajo pro hän/se ajoi.
• The partitive forms of nouns with a final /o/ is sometimes -oo instead of the standard -oa. E.g. se ajo rekka-autoo, 'he/she drove a truck'. This assimilation is more common when the partitive form is followed by an enclitic particle. E.g. rekka-autooki se ajo, ['rek:a,?auto:kis:e'?ajo], 'he/she drove a truck as well'.
• Epenthetic/anaptyctic vowels can sometimes occur with various consonant clusters. For example: veleka pro velka, silimä pro silmä, helevetti pro helvetti, hölömö pro hölmö, etc. The anaptyctic vowel is identical with the vowel of the preceding syllable.
• Word-final /n/ before a long pause or a vowel can change into a long or short glottal stop, ([?:], [?]). In written vernacular, the final <n> may be deleted. E.g., se o ollu [seo?:ol:u] pro se on ollut; se on tässä, but tässä se o [t&s:&se?o?/t&s:&seo?] pro tässä se on; se on mu [seom:u?] pro se on minun. Before consonants, a final /n/ assimilates to the consonant; e.g. kymmenen kirjaa [kym:eneNkirja:].