Except for the really irregular verbs (sum, eo, volo, etc. and their compounds), all the forms based on the first two principal parts are formed regularly -- it's one of the nice things about Latin. So in this case, the imperfect would be:
vetabam, vetabas, vetabat, vetabamus, vetabatis, vetabant
So the irregularity, if you want to call it that, is just that the third (and fourth) principal parts are formed in an unexpected way. But once you know the principal parts, everything else is regular.