My vote - at least three vowels are allowed.
Could the person posting increase if they wanted? Say it was "Piroppo," and I wanted to be evil, maybe get three strikes on vowels, I'd say
_ _ _ _ _ _ _
Strikes (0):
Vowels used: 0/5
To be clear, is Y included in the vowel restriction? My vote is no on that.
While thinking of rules, what about a minimum word/entry size of 4 or 5 letters? That three letter one a few days back was no fun for me.