Some letters aren't letters

[Torsten]

In any case, for me y is another character that cannot be mapped to a letter.

If one glyph cannot be mapped to a letter then the same is true for all other glyphs of the VMS. Let me explain this conclusion with [y], [o] and [a] as example.

[y] can replace [o] and [a]. Moreover (deleted - see next post) 
Within the network of similar words [y] is used instead of [a] as last glyph in a word. For instance [dar] without [r] results in [dy] or [okal] without [l] results in [oky]:

aiiin( 41) daiin  ( 17) odaiiin( 4) okaiiin(  4) qokaiiin(  2) kaiiin( 3) taiiin ...
aiin (469) daiin  (863) odaiin (60) okaiin (212) qokaiin (262) kaiin (65)
ain  ( 89) dain   (211) odain  (18) okain  (144) qokain  (279) kain  (48)
an   (  7) dan    ( 20) odan   ( 2) okan   (  5) qokan   (  8) kan   ( 3)
aiir ( 23) daiir  ( 23) odaiir ( 2) okaiir (  6) qokaiir (  3) kaiir (--)          
air  ( 74) dair   (106) odair  ( 5) okair  ( 22) qokair  ( 17) kair  (14)
ar   (350) dar    (318) odar   (24) okar   (129) qokar   (152) kar   (52)
ail  (  5) dail   (  2) odail  (--) okail  (  1) qokail  (  1) kail  ( 1)
al   (260) dal    (253) odal   (13) okal   (138) qokal   (191) kal   (23)
am   ( 88) dam    ( 98) odam   ( 6) okam   ( 26) qokam   ( 25) kam   ( 9) 
os   ( 29) dos    (  1) odos   (--) okos   (  8) qokos   (  1) kos   ( 3)
or   (363) dor    ( 73) odor   ( 8) okor   ( 34) qokor   ( 36) kor   (26)        
ol   (537) dol    (117) odol   ( 2) okol   ( 82) qokol   (104) kol   (37)
y    (151) dy     (270) ody    (46) oky    (102) qoky    (147) ky    (25)

For word types having an [y] the glyph [y] is used in 3096 out of 4148 cases (74.6 %) as last glyph whereas [a] is used as last glyph in 73 out of 3760 word types (1.9 %). For this reason the word [chedy] occurs 501 times whereas the word [cheda] occurs only once.

For [o] this rule is less strict. [o] occurs in 314 out of 6920 word types (4.5 %) as last glyph for words including [o]. For instance beside [chy] (155 times) also [cho] exists 68 times and the word [chedo] occurs two times within the VMS. (Deleted since both instances of [chedo] are most likely transcription errors.)

My conclusion is that [y] is used as spelling variant for [a] and [o]. This means if [y] cannot be mapped to a letter then the same is true for [a] and [o].