Even assuming the worst case scenario and Vegeta doesn't dodge the decay wave, there's no way it'll kill him. People comparable to Shigaraki are able to react and avoid being killed by giving up their limbs. At worst, Vegeta will have to sacrifice a leg or two and get pissed off.
Note: Visually, decay doesn't look like hax. Even Izuku thought it was just a shockwave and tried to push it back. So, I can see Vegeta being taken off guard, but I don't see how it'd kill him. He'd fly up and nuke the area. Actually, there is a nonzero chance that in that situation, Vegeta might just blow up the Earth for being injured by an inferior species. However, he could settle for nuking the country or half the planet if he isn't that angry. (Because he can't survive in space)
If he sees the decay as a threat, he will dodge it, regardless of who it comes from. However, I can't say if he'll recognize decay as hax or just a shockwave of destruction.
Also, this match up cannot happen.
Versus Thread Rules: "Any speed equalized match, in which a major reason a character loses is due to having a disadvantage against some speed value they usually wouldn't have a disadvantage against, may not be added to profiles.
As a result, winning a speed equalized match against a faster opponent due to a speed boost so large that it blitzes the opponent will not be added."
There's another rule.
"It is not fine to restrict abilities in a versus matchup, implicitly or expressly. Matches that are arranged this way should not be added to the character profiles, as they don't involve their full potential, and are only intended for casual entertainment.
An exception would be if the restricted ability/technique has a separate tier from the main one, and is one the character can consciously restrict themselves from using. In this case, the match can be added.
Tomura Shigaraki has a blitz level speed amp, which cannot happen as Vegeta is faster. That means the match cannot be added. You also cannot restrict Tomura's speed amp because his speed doesn't go into a separate tier.
This thread should be closed.