IIRC, VLAN tagging is a software-based operation. How intensive have you found it to be?
Have you found much of a difference in ROS vs. a hypervisor doing the VLAN tagging?
I myself am still in the learning phase of the CRS3xx devices, ie. haven’t used VLAN on this device yet, but here are some notes from the wiki pages https://wiki.mikrotik.com/wiki/Manual:CRS3xx_series_switches#VLAN :
Since RouterOS v6.41 bridges provides VLAN aware Layer2 forwarding and VLAN tag modifications within the bridge. …
Bridge VLAN Filtering configuration is highly recommended to comply with STP (802.1D), RSTP (802.1w) standards and is mandatory to enable MSTP (802.1s) support in RouterOS.
…
Note: Since RouterOS v6.41 all VLAN switching related parameters are moved to the bridge section. On CRS3xx series devices VLAN switching must be configured under the bridge section as well, this will not limit the device’s performance, CRS3xx is designed to use the built-in switch chip to work with bridge VLAN filtering, you are able to achieve full non-blocking wire-speed switching performance while using bridges and bridge VLAN filtering. Make sure that all bridge ports have the “H” flag, which indicates that the device is using the switch chip to forward packets.
…
These devices don’t have switch chips.
Not necessarily. Lots of switches out there do in hardware.
Which devices?
OP mentioned hypervisor in initial post. So I guess he’s asking about CHR and indeed CHR doesn’t have switch chip.
As no instant response came, I’d guess the OP has already tested that on his own by now 
If so, can you share the result, please?
I am indeed looking at x86 and CHR platforms.