Yeah, I've wondered this myself. Why didn't he just take it from Cobb Vanth? The only thing I could come up with that makes some sense is that he had only just arrived in the area before he saw Mando leaving with the armor. Presumably Fett made his way back to Slave I, whereever it was, and followed Mando's trail before catching up with him on Tython.
I agree they could be setting the stage for Fett's permanent demise to hand off Slave I to Mando but the only thing that makes me think that isn't in the cards is that it seems rather silly to bring back a popular "dead" character and then turn right around and kill him off again. If they just wanted Mando to have Slave I, they could've easily had a scenario where he finds it (either abandoned or in the hands of some random nobody) and have him claim it, leaving Fett still dead on Tatooine.
