I'd say he was an antihero. He was fighting the killers of humankind, but without typical hero qualities. Does he slip into being an antagonist at any point in the Android/Cell Saga? You could say he did when trying to help Cell to transform and directly opposing Krillin and Trunks - but I'd say no, because he still had the same main goal as the Z crew, which was to defeat Gero's weapons. He wanted to face Cell's strongest form, but it doesn't change that he wanted to kill Cell. He'd be an antagonist if he wanted to help Cell to defeat everyone or if he was helping Cell to transform while letting him kill loads of humans (not just Android 18, who was an enemy anyway). Instead he's an anti-hero, because he has the same goal, but just has a slightly different idea of how to achieve it and maybe different motivations.
Was he a Z-fighter? For viewers he was a Z fighter when he turned up for Freeza and Cold's arrival, dressed like a flamingo. He'd fought alongside them already, only Z fighters turned up there, plus him having a kid with Bulma was revealed.
In-universe, against King Cold I wouldn't call him a Z fighter, since nobody liked him, talked to him or wanted him around lol, except maybe Bulma.
Against Cell I'll say he was at minimum a fringe Z fighter, because he'd already been training with Trunks, saved Goku and the others included him in their plans.
Against the androids I'm not sure. Did the others want him to be there? I can't remember if the Z fighters talked about Vegeta between the end of the timeskip and him arriving.