Plaster Walls Cracking above air 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.