Greetings, @Andrew Kaplan. 

Thanks for joining in on this thread. It's great to see a new face in the Community.

If after entering the vendor's name in the Purchase Order doesn't auto populate the email as well, then I suggest doing the steps above that my colleague provided about clearing browser data. This should help to eliminate any glitches or errors that are standing in your way. 

I hope this helps and resolves your issue. Keep me updated on if it works for you or if we need to do some additional digging. Have a fantastic day!