Vegeta is strong very strong, he’s way better than Goku when it comes to going on the offensive, not saying he’ll get the stupid theory of that offensive half of ultra instinct, he’s just more vicious and persistent than Goku. I don’t even think Vegito will be able to beat him even if he manages to show up, because 1 in Black Arc Goku won a blast struggle with merged Zamasu damaging Zamasu a decent deal, but still just couldn’t beat him until Vegito came out. Goku couldn’t even beat Jiren with a U7 spirit bomb and Jiren was only just looking at it.
2 Vegito blue only lasted 5 minutes, the two Saiyan warriors are much much stronger than they were back then so the fusion would literally be a waste of time and the fusion itself takes a toll on the users so that’s even worse.
So now let’s look at Vegeta he landed a good love tap and a ab crushing kick on Jiren pretty impressive given the fact Goku couldn’t even do that with Kaioken well actually Goku landed a couple hits but they were so fast and insignificant some people actually missed it. Back to Vegeta, he hit Jiren with a direct Final Flash that usually leaves its victims reeling, didn’t even scratch Jiren or even leave him a little breathless, hell his clothes didn’t even singe a little bit. What Vegeta got at the end was basically a soft Hakai leaving him almost dead. So can Vegeta beat Jiren? No. Can Goku beat Jiren? No. Can Vegito beat Jiren? No. Can a coordinated double team of Two Saiyan brothers in battle with enough power from both? Hell to the yes. Vegeta doesnt need the offensive part of ultra instinct because he’s already a stronger offensive fighter than Goku I said offensive not overall. The only thing that Vegeta would fall to Goku is the fact that Goku is a genius when it comes to combat, Goku isn’t an idiot he’s just socially blind because he lived in the middle of nowhere and hit his head as a baby. Sorry for the long read lol but just think, Vegeta wouldn’t be getting a power up if he wasn’t gonna be a factor in Jiren’s defeat. And so die hard vegeta fans won’t complain about Goku’s ““Ultra Ass Pull”. But until we see how Jiren’s defeat is at the mercy of the writers.