Are the VTL drives directly attached to the NetApp filer via fiber or are you using NDMP-over-IP to go through the media server for these backups? Also, in the backup selections of your backup policy have you specified a path that is either the root of a volume or the root of a qtree? (these give the best performance when performing NDMP backups)
If you have a NetApp NOW account take a look at the following links:
https://now.netapp.com/Knowledgebase/solutionarea.asp?id=kb18468
https://now.netapp.com/Knowledgebase/solutionarea.asp?id=ntapcs17121