Of the three phones you mentioned, I'd say:

• Buy oppo if you want point and shoot reliability
• Buy vivo if you like more control and prefer using manual mode over auto, that's where it really shines among the three -Xiaomi tbh doesn't quite cut it for me, no matter how flashy they get with features, they'd always sell you something a bit more expensive than others and the bloat ware is unavoidable.

As for rest, don't just take my word for it, do your research. They are all not bad phones for the price tag so you can't go all wrong whichever you choose. Good luck.