If Vengeant Earth targets a creature you control, Vengeant Earth will overwrite any previous effects that set the creature’s power and toughness to specific values. Effects that otherwise modify the target creature’s power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

The target creature or land will keep any abilities it previously had.

If the resulting creature attacks, the defending player must assign at least one blocker to it during the declare blockers step if that player controls any creatures that could block it.

You may target a land that’s already a creature. For example, if you target a land that’s also a 0/0 creature and has three +1/+1 counters on it, the resulting land creature will be 7/7.