Well it' a kludge but it seems to work... Using the Librarian VIs (VI.LIB\utility\libraryn.vi), you copy the VI into a temporary LLB. Then you can get the VI version with Librarian Get Info.vi.
I wonder why the version number is not available when the VI is not in a LLB since obviously the version number can be retrived after the VI is copied in the LLB.