what is running VSS on my server
I believe that if you run native SQL Server backups (using T-SQL BACKUP) instead of DPM protection agent-based SQL backups, the SQL Server VSS Writer service must not be used on your SQL Server database folders.  (The same problem is true for other third-party backup tools.)

This DPM/VSS service is not used by native SQL backups. If you prevent the DPM protection agent backups from triggering SQL Server VSS snapshots, this will protect your chain of log backups.

If DPM/VSS does continue to backup the SQL Server files, then the log chain is broken, because there are two tools (T-SQL BACKUP and DPM/VSS) backing up the log files.