I think the tape wasn't done well that is along the crease of the vaulted ceiling. Cracks are starting to appear along the seam. Any advice?

Remove, retape, refloat, texture and paint.

That's about it aside from live with it. I have a couple of small portions where mine is doing the same.

If it's moving enough to crack, it probably will move that much again, even with a perfect tape-and-mud job.

I'd cut the entire joint and refill it with a paintable flexible sealant.

Even better would be a bit of trim to span the joint, but fastened only on one surface of the two.

My first choice for such plane changes is to put a lighting cove in and kill two birds with one stone. This is not always practicable, sadly.

You've got a few options for this kind of crack.

The first is the aforementioned tape and refloat. I'd add some screws near the seams too. This may not be practical if your drywall was textured as getting the texture to match can be a bit of a trial. If you do refloat, then make sure they're not using timed-dry (aka 45-minute or 90 minute) mud... have them use the ready mix stuff. They won't like it because it means multiple trips for them 'cuz the stuff takes 12-24 hours to cure before you put the next layer on. It seems to hold up better over time and be less subject to changes in humidity and temperature though.

The next is filling the crack with a paintable sealant. I like latex caulk called "big stretch" for this application. Use a generous bead and leave it a little proud. Then you can paint it and it should hold everything together.

The last is putting trim up to hide it. Believe it or not, this is what crown moulding is made for. Might be difficult if you've got a vaulted ceiling with 45 degree corners; otherwise, it's a viable option.

I second the Big Stretch method as long as the tape is not popped loose

usually happens when the float is too thick to begin with.

You should see the ceiling in my bedroom. It pisses me off everytime I look at it.

I guess it is time to redo it.