There's tons of possible reasons you're struggling in vMA, but it's not because you're on a Stamina NB. I think Stamina NBs are just as strong as any other stamina class in the arena if not a tad stronger. Ofc, you're constantly fighting hordes of magicka NBs for leaderboard spots, but that's neither here nor there...
Maybe post your build and overall strategy along with what's killing you? There's lots of knowledgeable players willing to help, but need more info.