Looks like you are running a NetBackup 6.0 Master server, are you running any maintenance packs?

You might want to try to determine what process is leaving the defunct processes.  My bet would be bpduplicate or the less unlikely vtlrun process.

Create the log directories, bpduplicate, vault and admin under the /usr/openv/netbackup/logs directory.

Increase verbose logging level to 5 in the bp.conf and restart NBU daemons.

Once you have some defunct processes, try greping for these process id's in those 3 log directories.

This should atleast start you on the right path. 

Yes, you are right. They really appear to be bpduplicate processes that abnormally terminated with status code 50. According to bpduplicate log the termination was due to ltid failure on the media server.

I have also noticed that in spite  of a vault job sometimes appears to be terminated with status code 150 in Activity Monitor, it is still goes on. It looks like the second try of vault job.

I wonder what causes the fake termination of vault.

I would download the Vault patch and read the readme file.  This file will contain ETRACK's for identified issue, if your issue is listed then the direction you want to take is to upgrade your Master and Media servers.

Otherwise you may want to open up a support ticket with SYMC and provide your findings.  Might be an issue someone else has seen in which SYMC support may be able to provide an updated binary.