1: Well it depends on your setup.
If your client can handle oversized frames you should not have to change the MTU.
If the driver of your em-NIC's cannot handle oversized frames, you would have to change the MTU on the other side of the link, so no oversized frames arrive at your end.
2: The problem is, that VLAN-tags add 4bytes to every frame.
–> frames can become bigger than the allowed maximum.
Most drivers can handle this, but some have problems.