[CentOS] OT: virtual host for apache/tomcat servers

I hope someone here can answer my question so I don't have to join
another mailing list, on too many as it is...

I have the following installed:
httpd-2.2.3-43.el5.centos.3
tomcat5-5.5.23

I have a small jsp app that I can hit through apache like:

http://www.mydomain.com/tomcat/myapp.jsp

So tomcat in the url above is the apache proxy address for the tomcat
server. 

I want to setup a virtual host in apache to automatically load this app
if I type:

http://myapp.mydomain.com

I'm having trouble figuring this out. If I try to setup a virtual host,
apache complains that /tomcat is not a valid DocumentRoot. Looking at
tons of docs, but not seeing the solution. 

Any help is appreciated. 

Thanks,
James