There are blocks designed to address this sort of trouble. They are simply aluminum or steel blocks drilled through with the appropriate diameter hole and then split. In use they are clamped to the tube and turned to "iron out" the dent. Paint obviously suffers from this process. Result are rarely if ever perfect.
The cheap version is just to put the frame back into the vice. Results will likely be even less perfect.
I'd probably leave it alone if I were you. If anyone asks you can tell them it is "multi shape" tubing. It's technically not as strong as it used to be, but that likely means you've reduced the safety margin from 500% to 430%.
And now you know not to clamp frame tubing in your vice!