Hiya,
The virtual servers are to be considered like any other server on your network. So in regards to VLAN's it's the same considerations, pro's and con's about using the VLAN. You get added segmentation(Broadcast control, Bandwith throttling etc.), but increased
administration(VLAN management, routing between VLAN's etc.)
So while VLAN is NOT a requirement, it could be useful, but mainly dependent on your infrastructure. The more servers you have, the more useful it usually gets. I would not consider VLAN unless one of the following statements were true:
1: More than 20 servers.
2: One or more of my servers relied heavily on broadcast traffic. (Windows NLB as an example)
3: Requirement of traffic segmentation, either based on security or bandwidth control.
That said, my perspective is exclusively from server administration side. I have not and do not engage with network administration, configuration nor design in my daily work.