No way. Vegas aren't prioritizing the re-signing of some flakey one-dimensional goal scorer deadline dud over the multitude of other much bigger cap issues they have on the burner?
Anyway...
Mantha feels like a prime candidate for a "pump and dump" routine for some bad team. Give him a ton of offensive minutes, top powerplay time, the best linemates you can find for him. Let him score his goals...and then try to flip him at the deadline to some sucker because he's a "big goal scorer". Which is technically true, but functionally irrelevant.
Just have to make sure that it's a 1-year deal at a cap number that is palatable on the "secondary market" at the deadline at max retention. For the teams that miss out on or don't want to spend on the "big fish" but still want to look like they "did something". Whatever his cap hit ends up being in UFA...you may as well just start looking at it as what his value would be at half that price at the trade deadline.