When you say "rumored," DC, you're opening up a very wide door, because no one knows for sure who pulled the trigger. In descending order of liklihood:
1. Luciano and Adonis ordered the hit because they suspected Bugsy and Virginia had stolen some or most of the $5-6 million they fronted for the Flamingo. The triggerman may have been Frankie (Mr. Gray) Carbo, who was one of the triggers for the Harry (Big Greenie) Greenbaum hit that Bugsy planned earlier. Carbo, who later ran the fight rackets for the Mob in NYC, would have had a personal incentive to kill Bugsy as well: eliminate someone who could have ratted him out on the Greenie hit. The movie "Bugsy" implies that Mickey Cohen may have pulled the trigger, but I doubt it.
2. LA Mafia boss Jack Dragna organized the hit in revenge for Bugsy's muscling in on his rackets. He got permission from Luciano to proceed. If that were the case, it'd validate the claim of Eddie Cannizzaro, who (much later) claimed to be the triggerman.
3. The Berman brothers did it--they were among the dozens of investors to whom Bugsy sold thousands of points in the Flamingo, but they had the muscle to do something about it.
4. Virginia Hill's brother, Chick, did the deed. Chick supposedly hated Bugsy and was known to own a .30 MI carbine, the type of rifle identified as the murder weapon. But so did several hundred thousand others.
5. The Chicago Mob ordered the hit because Hill was their eyes and ears in the West, and Bugsy abused her. I doubt it--she gave as good as she got, and probably abused Bugsy.

Assuming one of the above is the answer you were looking for:

Who assassinated Italian anarchist Carlo Tresca on Vito Genovese's orders?