Basically we have some background process which updates "table1" and we don't want the application to make any changes to "table1" while vacuum. Vacuum requires exclusive lock on "table1" and if any of the background or application is ON vacuum don't kick off.
VACUUM FULL needs an exclusive lock, the regular one does not in 8.1. It's one of the reasons FULL should be avoided. If you do regular VACUUM frequently enough, you shouldn't ever need to do a FULL one anyway.