Yes, it's prolapsed vent. It's caused most likely by pasty butt clogging the vent, then by constipation as the poop backs up in the intestine. The chick is straining to poop, and that causes the vent to collapse.
You need to get some mineral oil or coconut oil into the chick to break up the clog. Then you need to use Preparation H on the "bulge" to shrink it. Don't try to stuff it back inside. Coconut oil on the vent with Prep H will help heal it.
Hold the liquid oil under the chick's beak and it may take a big drink on its own. If it is too weak to do that, try squeezing a few drops of mineral oil into the right side of its beak.
You will know for sure the chick is constipated if it chirps loudly and madly while squatting and straining and only passes a tiny poop or very watery blob. It's in a great deal of pain. Either treat it or cull it so it doesn't have to suffer.