ok, so maybe this has been said before...
About transitioning - I'dd rather only have to press RT and hold it down to climb on a wall or ceiling or whatever...RT would be used as a "stick to wall" button, like in AVP1 and 2. Simply letting go of the RT would make you fall back to the floor.
about the enter vent - obviously, if the vent is on the ground or on a wall that you are currently walking on, I believe it should be more simple if you could walk in and out freely. But, looking at the video, if youre on the ground and a vent is on a ceiling, its way easier to enter it by tapping A button than it is to jump, stick to the ceiling, then walk in it freely. I think the A button scheme is a "aim-shoot" way of getting in the vent without doing the "jump-stick to ceiling-enter vent" manually. Having said that, if you are walking in a cramped space, and want to walk on the other side of a hole in the vent shaft, then it would be impossible to do so if you would freely enter or exit the hole without pressing a command.
In conclusion, any opening steming out of a wall (vertical wall) should be freely accessed or exited, while any opening from a floor (unless you want to drop in the vent, not drop out of it) or ceiling should be a VERY QUICK animation.
Just my opinion.