Joule vzn 80 or 100 for merlins?

Stargates and VZN80 do not have enough juice to drive the Merlin floorstanders. Heavengates or VZN 100 monoblocks or any high quality OTL tube monoblocks with at least 75wpc are your best. Mesa Baron would be a nice match with the Merlins (if you can find a used Baron).

Are you saying that a OTL has to have 75 watts or more to push the VSM's and not so with non-OTL amps?

Your room size may also be a consideration.
Given the fact that many Merlin owners use the 60 watt Berning to drive their speakers would suggest that for most people, this power would be sufficient.
Also, the difference between a 80 watt amp and a 100 watt amp is about a 1dB increase in volume. Not much difference there. However the VZN 100 is a mono block vs the VZN 80 stereo amp.
If you plan to buy used, the $1000 difference in price would have me leaning toward the VZN 100 mono blocks. But that's me.