MP4 is supposed to be the "best" MP for 6.0 currently.
You could always read throug the E-track listing for the MP.
http://support.veritas.com/docs/285941Maintenance Pack NB_60_4_M.winnt.intel.exe provides fixes to the Veritas NetBackup (tm) Enterprise Server / Server 6.0 on NT/2000/2003/XP Professional server and clients. It also contains fixes for NetBackup Add-on products and Database Agents.
What kind of VSP issues are you having?