Acme.com? Or Acme.com/buddypress/acme.html?
I can’t wrap my head around how this would work with “true” DNS.
In any case, a simple way to do it would be to just instruct the owner of Acme.com to set up an iframe to the relevant page on Community.com.
This isn’t exactly a “rewrite” but will (appear to) accomplish the same end result.
MX records are related to email. The “MX” stands for “mail exchanger” …so no, this wouldn’t have anything to do with MX records.
This would be handled by the same aforementioned iframe, so no additional steps are necessary here.
However, it sounds like you want the URL to update in the address bar too. With an iframe, the URL would still be Acme.com while the relevant Community.com page loaded within.
I suppose you could load a different type page depending on the referring URL. An iframe if you’re referred by Community.com, a straight redirect if you’re not (i.e. accessing Acme.com directly).
I’m going to trust you have a good reason to do something like this. My initial thought is this seems to be a strange and confusing way to handle things, no matter what path you use to get there.
Imagine the user confusion if they click a vendor profile page, find themselves on a new domain, but appear to be on the exact same site.
Have you ever seen another site that does anything like this? If not, maybe there’s a good reason why.