Cut Floor joists for HVAC
Yeah, that wasn't very smart of them. Fixing the stucco is likely a bandage for the issue they created here.

You'll have to move the vents and then sister a beam on both sides of the cut. First, get two construction jacks to raise the beams back to level. Next, sistering involves buying some new boards that are the same size as the cut ones. Then you construction adhesive the new boards and pass bolts through all 4 boards to hold them together. You'll probably want to fill the void the duct work left as well.

It's worth consulting with a structural engineer to ensure nothing else is wrong.