I got your point but I dont think that make sense trying to mesure this traffic. I think make sense measuring traffic when we are talking about IN or OUT. When you are dealing with Layer 2 Vlan, this is not IN or OUT, this is just a domain where traffic spread around. Do you get me?
That´s why Interface vlan has RX and TX cause Interface vlan means In and/or out from the device. What make sense to me is measure the traffic In or Out in a specific interface inside this Layer 2 vlan.