Reverse proxy

Can someone please comment.

Apache server A is a physical server on my network. I has three virtual 
hosts serving three
different websites. This appears to be working correctly.

Introducing Apache server B
Apache server A also acts as a reverse proxy to Apache server B which is 
another separate server with a static ip, and
acts as my mail server.

There are two virtual hosts defined on Apache server B, one is 
mail.domain.com and one is postfixadmin.domain.com

My question is can Apache server A route (via reverse proxy) to the two 
virtual hosts on Apache server B.

At this point it's close to working, but my postfixadmin.domain.com is 
having it's document root directed to
virtual host mail.domain.com, instead of postfixadmin.domain.com

I've spent hours checking my virt host configurations. Is there some 
other setting outside the virtual host configuration that
is allowing the DocumentRoot to be hijacked?

Can someone please confirm my setup is possible?
Regards