Shambling Vent will be a 2/3 land creature - Mountain Elemental with lifelink and the ability to tap for one red mana for the rest of the turn. Setting the land's subtype will get rid of its previous land types, but it won't get rid of any abilities granted to the land by other effects, like the Vent's own ability. It will still be an Elemental (since changing land subtypes won't affect creature subtypes the card has). It will have lifelink since that was given to the land by other effects.
305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text, its old land types, and any copy effects affecting that land, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Setting a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.
Nathan Long
Magic Rules NetRep