Have you had the truck for a long time and recently noticed it to be sluggish, or is it a recent purchase??? There's a lot to guess at here. The first thing I would say is that the 5.2 is slightly under-powered for a very heavy 2001 Ram. To see if the plenum gasket is leaking, remove your air filter assembly, open the throttle body, and look down into your intake. If it looks wet, it's leaking. As for the vents not blowing when you step on the gas, it might just be a bad blend door actuator. Those trucks are notorious for the blend door going bad.