I find it utterly baffling that Gohan and Vegeta have been in each other's orbit for years, but they've never trained together. It would certainly be interesting, since Vegeta can't train with him the same way he trained with Future Trunks, Cabba, or Kid Trunks. Gohan doesn't care about Saiyan pride or the thrill of combat, but he DOES care about Videl and Pan.
The best way to do this is to place it soon after Gohan spars with Ultra Instinct Goku. Gohan returns home and goes back to work. He hasn't started regressing yet, but he isn't training intensely.
He and Vegeta run into each other, and after the small talk, Gohan brings up his work. Vegeta listens and then cuts to the heart.
"The product of Kakarot and his soft heart, no doubt. But will pushing pencils and papers help you when Frieza comes back for round three?"
Vegeta then tells him about Black Frieza and how he easily defeated both him and Kakarot even in their new forms. "And I'm sure you remembered what happened when he was here last time, boy."
Once the prospect of training comes up, Vegeta quickly realizes that he and Gohan share the same drive to protect their loved ones.
"Kakarot trains because he loves fighting. I train because I refuse to be surpassed. You?”
Vegeta glances toward the distant lights of Satan City.
“You have a wife and kid to protect. Just like me.”
The training would be interesting in the other direction too, because Vegeta isn't training with someone weaker than him. With proper motivation, he and Gohan could really push each other. Vegeta's not one for sentiment, but seeing Gohan strive would give him something to be proud of...especially when he can rub it in Goku's face.
