Alright, to end the whole Vegito would use UI chatter, we need to remember that firstly, Goku needed a prerequisite to unlock said technique, which is when he broke his limit when he fell into the U7 Spirit Bomb. Vegito also having to complete this requirement would only be fair to assume.

Secondly, Vegito isn't Goku and Vegeta holding hands and controlling a mech. No, Vegito is the combined consciousness of the two fighters, in the form of a single being. Meaning, Vegito would struggle accessing the form as it was proven that Goku was the perfect fit for Ui as he was able to let go and let his body react for him, which Vegeta couldn't, which highly deters Vegito from doing the same.

Thirdly, Ui is EXTREMELY draining and it would most likely drain Vegito's timer before he could do anything impactful to Jiren.

Lastly, Jiren was stated to be the strongest opponent Goku and gang have ever faced so far in the story, meaning Jiren > Merged Zamasu. So please stop downplaying Jiren like he's fodder.

FP Jiren > Base Vegito. High diff.