Tomcat assistance desperately needed

we have a Turnkey Tomcat installation using basic config i.e. / = localhost

we need to add a virtual host, such that new hostname 'new.newdomain.com' points to 

existing webapp located at  /newdomain (in webapps list)

this is a 2 line change in Apache httpd.conf but the virtual hosts manager cannot be made to work correctly and we do not know what we are doing wrong. Help??