As was stated, mod_jk may work just as well, as that's what it is designed for. Otherwise, if you are set on using mod_prozy, it should be fairly straight forward, although I do typically provide it a URL to redirect off.
For example, in your situation, you would have the following in the configuration for your virtual host:
ProxyPass /zimbra
http://mail.MYDOMAIN.COM:8080/zimbra
ProxyPassReverse /zimbra
http://mail.MYDOMAIN.COM:8080/zimbra
Dont forget the /service proxy, otherwise you will not be able to log in.
ProxyPass /service
http://mail.MYDOMAIN.COM:8080/service
ProxyPassReverse /service
http://mail.MYDOMAIN.COM:8080/service
This is how I currently have my Zimbra installation set up; I proxy all requests to my Zimbra server from my primary webserver.
Good luck.