Waiyyakum literally means “and to you”. It is a form of dua`a though I haven’t come across it as being sunnah. In any case, when someone pray for another with ““, there’s nothing wrong in praying for the same for that person, in return. That can be done in a number of ways:
- to you as well
- and to you as well
- same to you
- May Allah reward you with good
Or one can make any other dua`a if one wishes to do so. However, it might be good not to use waiyyakum all the time, since it could give the impression that it is something that is explicitly sanctioned as sunnah and it can end up being misleading (as I’m finding out now).
Allah knows best.