> I can't see PIE having less than 3 vowels. Even then 4 makes more sense.
However many vowels you give it, it is extremely odd as a language, in that
it seems to use only e (or o in relatively predictable situations) for full
grade stems.
> IMO, *ei and *eu are the result of stressing *i and *u,
Now there's a wonderful idea! Pre-PIE had the vowels **i, u, r, l, m, n,
but speakers did what we know has happened several times, at least in
Germanic:
/i/ > /&i/, /u/ > /&u/
You can hear this change today both in Birmingham, UK, and in Bavaria,
Germany, and it underlies the English Great Vowel Shift.
However the system **i, u, r, l, m, n is itself odd - there should be a low
mid or low front vowel in there.
Peter