At Night, normally only the Scenario Defender sets up onboard, and can set up concealed, regardless of terrain, per E1.2.
I don't actually see that applying in VotG for the Scenario Attacker - even if setting up onboard - they do normally enter Cloaked.
Not sure if the VotG rules say the can be cloaked even when setting up onboard. My guess would be that's the intention.
Of course, I could have missed something....