Bees in a vent line are a new one to me. I was once on a boat that was stopped by mud dabber wasps filling the vent with mud.
I'm a bee keeper in my other life and I can tell you that once bees decide they like a spot, they often return. I doubt you'll have the problem.
Your bees were probably just a swarm looking for a new home. My guess is that the Queen thought that your vent was a good place to hide while her scouts found a new home. The rest of the bees would have clustered around the opening to protect her.
Of course if the tank is empty, they may be going down the vent into the tank and building comb. That's going to bee some really bad tasting honey.