Let's say I control a Shambling Vent and then cast a spell with Awaken 3, targeting the Vent. The Shambling Vent becomes a 0/0 Land Creature - Elemental with 3 +1/+1 counters on it, no problem there.
I then activate Shambling Vent's second ability, turning it into a 2/3 white and black Land Creature - Elemental with lifelink and 3 +1/+1 counters on it. I know that the new P/T setting ability overwrites the first so I effectively have a 5/6 Land Creature, still no problems here.
However I'm not sure what happens when the turn ends and Shambling Vent's ability wears off. Does it go back to being a 0/0 elemental or does it stop being a creature entirely?