It's important that Watford don't lose by too many against Man City.
Watford are currently +4 goals compared to Villa.
If Watford can keep it down to 1-0 or 2-0 tonight then that's a good result.
Any Villa loss against Arsenal will drop them below Bournemouth on goal difference and make it unlikely that they can overtake Watford on Sunday - even if they win.
If Watford lose 4-0 or 5-0 etc. and Villa lose only 1-0, then Villa will still be in with a chance.
If they can beat West Ham on Sunday, Watford lose, and Bournemouth only draw, then Villa will stay up.