I honestly never heard of Vivo or Oppo phones, but I wouldn't choose a phone for the way it looks. Sure, iOS devices sure luke neat and professional, but if Oppo had slightly better specs, that's what I would choose. Hmm, well to be honest, I would just pick whatever is the cheapest device, if you say both of them are relatively the same device.
But could you post one of each of these devices here, along with some specs for comparison? It's hard for anyone to give their their opinion if they don't know what they have to compare.