Two things really, the first is that you can't change the version of vB your hack is deigned for, it doesn't appear in editpost.
The second is that IMO threads created in the vB3 forum should only be able to choose vB3 prefixes and vice-versa. Take
this thread for example, its a hack for vB2, in the vB2 forum, but the prefix says vB3 and theres no way for moderators or the author to change it