Does anyone know a workaround to exclude files from VNX NDMP backups?

With Avamar, there is no workaround for this.  Per the Avamar NDMP accelerator guide:

Include and exclude lists

You cannot define dataset include or exclude entries when performing an NDMP backup of

VNX, VNXe, and Celerra storage systems. You can define exclude entries for NDMP

backups of an Isilon storage system. The EMC Avamar Operational Best Practices provides

details about how to use exclude lists with Isilon backups.

Avamar doesn't process primitives from an NDMP variable, therefore, you cannot exclude files.  Unfortunately, your PST files will still be backed up when you run your NDMP backups.

Let us know if that helps!

Karl