The answer is "Probably, but not very quickly."
The energy of a collision comes from two sources, the kinetic energy of the collision and the gravitational energy. If Mercury were to impact Venus, even if it began by creeping up on it very, very slowly so as to have as little kinetic energy as possible, it would fall into Venus's gravitational well and release gravitational energy enough to melt both bodies. The resulting mess would take ca. 10-100 million years for a crust to resolidify.
The resulting body could evolve towards habitability. One theory of planetary development says that active plate tectonics are critical to habitability. We know that new rock absorbs large amounts of CO2 -- new volcanic rock in Indonesia has been estimated to absorb a quarter of the CO2 released each year. So the continuous exposure of new rock by plate tectonics may be a major force in preventing the runaway greenhouse effect that has made Venus uninhabitable.
We don't know for sure how plate tectonics starts or ends, but if the resulting planet had a particularly vigorous case of it, this might compensate for its closeness to the Sun.
On the down side, Venus is already short of hydrogen and thus water and a collision with Mercury would eject Venusian volatiles into space and Mercury would not add any new, so the resulting planet might be rather barren.
In any event, whatever happens, it's going to take a very long time for the resulting body to cool enough to be livable.