Close, but actually, it is easier than what you have setup.
Keep the original entry, but add additional ProxyPass directives to cover the directories you care about, e.g.
ProxyPass /site1 http://1.2.3.4:18000/
ProxyPass /site2 http://1.2.3.4:19000/
ProxyPass /site3 http://1.2.3.4:20000/
and the proper Reverse entries as well.
However, depending on how many of these redirects there are, you may want to use the mod_rewrite module, or just install a number of redirects.
Yet another way is to add a number of DNS entries such as site1.foo.net, site2.foo.net etc. and redirect all requests to those hostnames. Then create a vhost setup for each site.
