Which is probably the name of one of these idiots in Jabba's palace, but don't be an asshole.
Yes, Vader is the one who murdered the Emperor in the first place, so maybe his claim to authority isn't super legal, but the only witness to that crime is Luke Skywalker, who isn't exactly eager to see Imperial justice get served. Besides, it's not like anyone would be challenging Vader for command in the middle of a giant space melee. The Empire's too busy getting their ass handed to them by a pack of feral teddy bears to comb through the Death Star's detailed succession bylaws.
The point is, Vader is now in total command of the Imperial fleet and has ostensibly returned to Team Blue Saber. He could just grab a radio and legally order all Imperial forces to cease fire and stand down, then broadcast surrender to the Rebellion. Doing this would save thousands of lives at Endor -- both Rebel and Imperial (and potentially scraping this moment from our childhoods) -- and would pave the way for an orderly, legitimized transfer of power. This final heroic act would ensure a noble, selfless end for the fallen Skywalker.
As long as everyone is willing to overlook certain things.
Instead, he sees the death and chaos and panic going on all around him, and he ignores it, because it's more important to get in a nice chat with the son he was trying to kill ten minutes ago. Hey, Skywalkers: It's great that the family feud is over, but you maybe wanna wrap up the space war that's killing all your friends before you start shooting that Hallmark commercial?