I'm seeing this same problem. I migrated my Win7 VM from Parallels 8 and I wasn't seeing any weird NetBIOS traffic before. It definitely seems related to the vmnat process. For me once the NetBIOS chatter starts (usually within a few minutes to a half hour of starting my Win7 VM), it generates about 80k/s of traffic!
If anyone else is seeing this problem, please reply. For any VMware guys seeing this, let me know what I can do to try to help debug this situation more.
-Arion