My house was built in the 1950's and have plaster walls. I am in the process of repainting and have noticed that the plaster above the baseboard air vents has cracked above almost every vent in the house. What could cause this? Moisture in the air coming out the vents?
That probably happened in the 1950s, not long after the house was built, especially if you have wood lath and not gypsum panels. The corners of the cutout for the vent create weakness, and normal seasonal house movement results in cracks.
Rub some painter's caulk in there before you paint. Doing more than than will probably just make it look worse unless you completely redo the drywall. It's one of the traits of older homes.
" What could cause this? "
My guess would be the furnace was turned off and the house got cold....real cold.
Then someone turned the heat back on and the flow of warm air caused materials to expand around the vents.
Too bad they didn't reheat the house slowly and incrementally (assuming my theory is correct) and avoided the problem.