NDMP uses the dump program running on the filer where the VMware system are stored. Dump level 1 (incremental) looks at the VMware files modification date and if the file has been modified backs it up. Dump is not VMware aware and cannot backup the changes with the files.
To backup VMware system using full and incremental backups use the NetVault VMware plugin. The plugin will do the equivalent to incremental backups if you use the Change Block Tracking (CBT) option. Alternatively you could use the file level backup option and this will enable you to run full and increment file level backup of the VMware system.
The plugin can be downloaded from the Technical Support Website: