I'll admit that i don't know much about Vance at this point, but on the surface i dont' think that he's a bad pick.
And while I think that normally nobody cares abou the VP picks, i thihk that the situation with President Biden healty/mental faculties/age and the generally negative perceptions of VP Harris's competence bring more attention to the VP picks this time around. And, while admittedly I'd like to see a Trump landslide, I think this race could still be close... so the VP pick may just matter.
Defintely a play to influence the race in the critical midweest swing states. He is youthful, and not a bad looking guy. My wife commented several times on the fact that he is the first candidate on the national ticket in a zillion years to have a full beard, and also how facial hair is back in style.
That youth and the fact that he had a tough go of it in his early life and his service in the Marines, I believe, makes him someone who can keep up with the pace of Trump from a physical work/mental energy standpoint. I think Trump has concluded that he will be loyal, and after Trumps first term, i think he (Trump) now has a concept of how backstabbing people in the DC circle of power can be. And, IMHO, Trump is looking for more than help with the campaign. I believe that he will expect more of VP Vance than to stand around in case something happens to him. Again youth and qualities brought out from his service in the Marines will be an asset here. Once on mission, I expect him bulldog whatever it is, for as long as it takes. Your 30's/early 40's is a prime time for making things happen through sheer force of will based upon your convictions. You havent had time to get real jaded or beat down into subscribing to the "this is the way things are, have been, and always will be" train of thought.
Vance as a pick is also right on message and follows a consitent theme that directly attacks the notion that the Republican party is simply the party of rich, white, racist, old men, and big corporations while the Democrat party is the party for youth protector/helper of blue collar America and minorities. Amber Rose and the Teamsters President , charlie Kirk {young), the frat bros ad the jewish students that got up on stage are in line with that. Additonally, like Amber Rose, he promotes the theme of an Anti Trumper who changed their minds.
Vance's family saga (poor, broken home, drug addicted mother, raised by granparents, joined the marines to escape..) should resinate with many. He has a degree from Yale, but you can't label him as priviledged. He definitely had a rougher start than either President Biden or Kamela or President Trump for that matter.
I dont think that it had a part in the calculus of selecting him, but it doen't hurt that Vance's wife, in addition to being a successful professional, is not lilly white. Her family only immigrated to the US in the 1980s.