If I activate Shambling Vent's ability after it has been 'awakened', what happens at the end of the turn?
Both Awaken and Vent's ability generate a continuous effect that applies changes in layers 4 and 7b – type and power/toughness accordingly (also some other layers which do not intersect).

611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects players or the rules of the game, for a fixed or indefinite period.
613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.

Awaken's effect is not &quot;overwritten&quot; by Vent's ability. Rather, Vent's ability is applied &quot;over&quot; it and takes precedence due to being more recent (timestamp rule).

613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp

Therefore, after Shambling Vent's ability wears off it will go back to being a 0/0 Land Creature - Elemental due to Awaken's effect.