I'd rather package vala such that there will be lua5.0.vapi and lua5.1.vapi. Then, packages depending on it would have to be fixed according to the relative new vapi files, which seems reasonable as debian ships different versions of lua.
If the aren't breaking changes between the two lua versions, it would have been possible to keep installing lua.vapi, as to have 3 lua vapis. In case vala breaks lua.vapi, then the old one can be kept in debian/ while updating only the latest lua5.1 vapi. -- www.debian.org - The Universal Operating System