OK I just found this statement (answered by a Symanetc employee):
OK, with AVVI agent, you will not be able to run log backup. Also, in order to run a log backup outside AVVI agent, you will need to perform a full backup using sql agent. AVVI backup is not recognized as a sql agent backup by SQL Agent. Hence, the limitation. So, the only solution, is to perform a full backup of sql database using sql agent, followed by log backup. If you do an AVVI backup, you might have to perform a full backup using SQL Agent again, so that log backup succeeds. Currently, this is the default behaviour of AVVI Agent in Backup Exec 2010
In this thread:
http://www.symantec.com/connect/forums/be2010-best-practice-virtual-ms-sql-2005-server