Short and long vowels
The short vowel i is the positional variant of the vowel i, and is evidenced in dialects of the Georgian language (Khevsurian, Tushian). It represents the positional variant
a) of the nominative case marker -i: k'ac-i (short i) (Khevsurian, Tushian)
b) of the aorist tense/aspect form marker  - e (-i seems to be its intermediate step and it is represented in the plural forms of the non-syllabic stems):  gavig-i (short i) (I understood) -  plural: gavig-i-t (ordinary i) (we understood) (A. Ch'inch'arauli, Xevsurulis taviseburebani [Specific Features of the Khevsur Dialect], Tbilisi, 1960). (Syllabic stems show -e in plural: gavak'et-i (I did) --  plural: ga-v-a-k'et-e-t (we did).

Positionally caused phonetic changes, in particular the process:  -i (ordinary -i) > -i (short -i), that is caused by the influence of the stress, have affected the morphological structure of verbs in the dialects where this phonetic process occurs. For more information: Morphological changes caused by stress: the iterative tense/aspect forms in dialects

Long vowels are always positional and  are formed in the following cases (long vowels are underlined in the text):
a) through fusion of identical vowels (on the boundary of morphemes): ga-u-k'et-a > gu-u-k'et-a > gu-ket'-a in Atcharian dialect, gu'ket in Tushian (he has done something for somebody).
b) to compensate for a lost sound (aghar > agha (never more), sircxvili > sicxvili (shame) in Atcharian
c) as a result of accent: sopeli (village), ushvele (help him) in Mtiulian, as well as in other dialects.
d) to render monosyllabic words disyllabic: hq'avs (has), dzmas (brother. Dat.), in the plainsmen's dialects of eastern Georgia;
e) in intentional lengthening of a vowel: didi (big), p'atara (little) with a stilistic emotive, expressive function.
(Besarion Jorbenadze. The Kartvelian Languages and Dialects. Tbilisi: Mecniereba, 1991, p. 42; Tevdore Uturgaidze. Kartuli enis ponemat'uri st'rukt'ura [The Phonemic Structure of Georgian]. Tbilisi: Mecniereba, 1976, p. 12-20).

Long vowels, formed as a result of the positional phonetic changing, may have a distinctive function; e. g. Two words uc'evs (lies with him) and auc'evs (He will raise something belonging to somebody) are distinguished by having the preverb a- in the standard Georgian and several dialects of Georgian, but in some dialects the merging of two vowels occurred and a one long vowel u has been formed auc'evs > uc'evs . Thus the length of the vowel distinguishes these two words in some dialects:

Gurian and Acharian dialects:

uc'evs (lies with him)
uc'evs (He will raise something belonging to somebody)

uc'evs (lies with him)
u'c'evs (He will raise something belonging to somebody)
(Note: From the other point of view, length in Tushian has no distinctive function. A distinctive function depends on the presence of an intensive off-glide after the vowel u in the word u'c'evs (Tevdore Uturgaidze. Kartuli enis ponemat'uri st'rukt'ura [The Phonemic Structure of Georgian]. Tbilisi: Mecniereba, 1976)

Some linguists reconstruct the long vowels on the level of the Pre-Kartvelian language (Tamaz Gamq'relidze and Givi Mach'avariani, Sonant'ta sist'ema da ablaut'i kartvelur enebshi [The System of Sonants and Ablaut in the Kartvelian Languages]. Tbilisi: Mecniereba, 1965) although wherever long vowels appear in the Kartvelian languages they are often positional (Svan). Vowel alternation  in one group of verbs is also considered the argument for this hypothesis: the series of  tense / aspect  I and the series of  tense/aspect II differ from one another  with the alternation of vowels  in the verb stem; e. g. the stem in series I: vocalisation e (v-drek' "I bend something") and the stem in series II: vocalisation i (v-drik'-e "I have been bending something"). Gamq'relildze and Mach'avariani reconstruct the short vowel  e (reducible e) for this group of verbs in the Pre-Kartvelian language while for another group of verbs where alternation e / i does not occur, the original long vowel in hypothesized to have been as in v-c'er  "I write" and v-c'ere "I have been writing"). Although the largest number of verb stems where such an alternation occurs have a specific phonemic structure, certain kinds of consonant clusters occur either in the beginning or in the end  (Parnaoz Ertelishvili. Zmnuri pudzeebis ponemat'uri st'rukt'urisa da ist'oriis sak'itxebi kartulshi [Questions of Phonemic Structure and History of Verb Stems in Georgian]. Tbilisi, 1970). Even later borrowed stems with such a structure undergo vowel alternation.

Mutative vowels usually represent positional variants of the complexes (in order to avoid diacritics, mutative vowels are written like this: o:, u:) : thus, -we  as in k'wercxi becomes ko:rcxi (egg); and -wi as in c'wima becomes c'u:ma  (rain).
Mutative vowels are found in the Ingilo dialect, with parallel non-mutative variants in Mokhevian,  in Samckhe-Dzhavakhian and Lower Kartlian dialects. In the Ingilo dialect, the influence of Turkish is noticeable.

Mutative vowels may undergo the following changes:

Mutative vowels may "disintegrate": atxovebs > atxoebs > atxu:ebs > atxviebs (will lend) where:
 -ove > -oe  -- as a result of the loss of a sound.
-oe  > -ue//u:e -- as a result of the dissimilation that has occurred in the complex.
(This phenomenon is found in the Samckhe-Dzhavakhian dialect).

Mutative vowels may lose labialisation: c'ver-i > c'o:r-i > c'er-i (bear); t'vin-i > t'u:n-i > t'in-i (brain). Such change is systematically attested in the Fereidanian dialect.

Mutative vowels may lose palatalisation: chven > cho:n > chon; shvili >  shu:li > shuli (son, daughter) (in Mokhevian, Lower Kartlian, Samtskhe-Dzhavakhian, sometimes in the Ingilo dialect).

 Mutative vowels sometimes may have distinctive function; e. g.:  go:zina (we were sleeping) _ gezina (you were sleeping, in the Ingilo dialect), but this is not a systematic phenomenon.

Vowels of intensive on-glide and off-glide are attested in Tushian  (ascribed to the influence of the Bats language): 'uambw (he has told), pera' (like). It is characteristic of expressive phrases in other Georgian dialects (in interjections: 'a, 'oh, ho' etc.)

Aspirated on-glide is evidenced in Tushian, Ingiloan, Samtskhe-Dzhavakhian, Lower Kartlian dialects; h is preserved in borrowed forms in those words also where the standard Georgian has lost it: hambavi (story), harami (unmerited), halali (deserved).

Palatalised on-glide is attested in Tushian, Ingiloan, and partially in Samtskhe-Dzhavakhian dialects. There are cases in which vowels of aspirated and palatalised on-glide are found  in parallel  in one and the same dialect, as in Tushian: hagr // jagr (yes, very well).

Labialised on-glide and off-glide are found in the dialects: ori - wori (two), omi - womi (war) (Imeretian, Gurian); k'aco - k'acow (man! Vocative) (Kakhetian, Kartlian).

Nasalised vowels  are usually found in the position preceding a pause, giving rise to the consonant n: c'aval-metki > c'aval-metkin (Gurian, Atcharian dialects).