I woud like to share an experience I just made. It seems as if VBR would not prevent "full" tapes from being marked as FREE if depending "incr" tapes are present.
I created two media pools, one called "test full" , the other one "test incr".
Then I created a "file to tape job" which writes to these pools and had it run daily.
I tried to mark the "full" tape as "free". After confirming the standard message ("Mark selected tapes as free...") the tape was moved to the "free" pool.
The incremental backups on the other tape turned useless because I killed the full backup (at least if this tape is overwritten).
I would have been glad receiving a warning about the depending incrementals.