Jon Mason [off-list ref] :
[...]

I wonder if this is related to the adapter breaking large frames into
multiple descriptors.  On normal (non-VLAN) frames, this happens at MTU
8169. I wonder if enabling VLAN and jumbo frames (a combination I never
tried) brings down the threshold to MTU 7200.  

Testing suggests that there is at least a size filtering issue (fixed
in netdev-2.6.git but not in 2.6.11.xx nor in 2.6.12-rc). It is unrelated
to hardware vlan support and happens a few bytes above the MTU on the vlan
device (which is set to the same value as the adapter). Of course different
issues could hide in the dark.

--
Ueimor