Not sure of the tonal differences, as I've heard neither. But I didn't know if you were aware that the 18W had 2 EL84s, and the 30W had 2 6L6s. So most likely there were some noticeable sonic, and feel, differences. My DS30's 6L6 section could be made to sound very Fendery with the open back 2x10 portion of the 1210 cab. I'm assuming the VM30 could get you into that territory, too, as long as you didn't push its volume knob too far past the halfway point (again, using my experiences with the DS30 as a reference point).
I would love to hear the VM30 with its higher headroom, but that may be wishful thinking due to its scarcity.