The paint will never sand down flush, there will always be a little lip which will look like a crater once painted, especially with a smooth ceiling and textured walls.

I wont see your ceiling cracking from my house, but I would cut it back at the edges of the crack, and use drywall mud or those little tubes of spackle and a putty knife to properly apply a patch. It's just paint cracking so you won't need much of it at all, just enough to create a smooth finish once sanded so go past the edges of the crack about 3 or 4 inches with the drywall mud.

Let it dry, sand it flat, do another coat if this is your first time because you'll make mistakes, let it dry, sand flat and paint.

Unfortunately, your new house is settling so cracks in mouldings and fixtures will be quite common for a few months to a year. Just ignore it for now or use it as time to practice cutting it out and redoing it cleanly.

One of these options is your best bet. While you're up there on a ladder redo that tape on what I'm assuming is some kind of access hole cut in to the vent, or if you're really feeling fancy install a small access door instead.