I didn't think Vegeta doing what he did was weird at all. He is smart enough to know that Goku doesn't need it, for one. And two, he would rather help in his own way by making sure Goku isn't vulnerable while he's charging up the spirit bomb. I thought it was very much in character. Vegeta has always been his own person and shows how he cares in different ways than others.
I get your point but that was their best shot against Jiren at the moment,so i have mixed feelings about his attitude.
Unless he has something under his sleeve,saying "nope,not helping!" in the most crucial moment is ridiculous.
It could've worked if he had remained silent or was interrupted because he saw Brianne trying to attack Goku.
In the end,it's just that single line of dialogue that pisses me off a little.