Disable vscan = Impact on performance is positive, because scanning generates additional I/Os and makes some accesses slower.
I see all different implementatoions. Some customers sa, we already scan everywhere else (PC, Firewall, Mailserver) and do not need to san again on the NetApp.
The other have rules and/or regulations that form^ce them to scan again.
There is no "one fits all" rule for this.
Peter