I master a campaign of Vampire and one of the players used Cloak of the Night (Obfuscate) to disengage during combat. I let him do it at first but I wonder if this works this way. After his round of fighting, can he can become invisible? Does he need to disengage before using it?
You can become invisible during combat, but using Cloak of Night is an Instant action that's not reflexive. If you are rolling to vanish from sight, that's what you're doing for your round that combat; you cannot shoot or strike and then try to use Cloak. That means it's best to disengage first, as the people who are fighting you have a chance to oppose you with Clash of Wills or simply penalize your roll by watching.