NLB in unicast mode only supports VLAN Tagging if the virtual network adapter supports dynamic media access control (MAC) address updates. To verify if your network adapter supports dynamic MAC address updates, please consult the network adapter vendor.
In unicast mode, NLB uses a shared virtual MAC address. The shared virtual MAC address is used instead of the physical adapter's MAC address for all communications to the load-balanced IP addresses (VIP). If NLB is not able to overwrite the MAC address of the virtual adapter provided by the network card manufacturer's driver, NLB will be unable to function properly.
Note Support for the VLAN Tagging technology, which includes the hardware as well as the driver which provides the virtual adapters and the configuration software for VLAN Tagging is the responsibility of the network adapter manufacturer. If problems that occur seem to be related to VLAN Tagging, Microsoft Support may request that you disable VLAN tagging while the problem is investigated.
For additional information related to NLB and virtual adapters see:
278431 Using teaming adapters with network load balancing may cause network problems
For more information on 802.1Q please refer to the IEEE standards