I have a plone zinstance set up through Apache Proxy on OS X Server 10.5. The server is set up with a single vhost on port 80, with Proxy & Proxypass directives to the Plon
Set up a static URL that will not be proxied but served from Apache directly, like this:
ProxyPass /static !
ProxyPass / http://localhost:8080/VirtualHostBase/http/server:80/Plone/VirtualHostRoot/
ProxyPassReverse / http://localhost:8080/VirtualHostBase/http/server:80/Plone/VirtualHostRoot/
Then configure /static to contain your static content.
Sorry, not enough information but I'll offer a couple of comments that might help point you in the right direction.
First, ProxyPassReverse is unnecessary as Plone already takes care of fixing up any self-referential urls. That is the point of the crazy url after all.
Another poster already showed how to configure ProxyPass to selectively bypass the proxy to Plone and serve from Apache directly.
Regarding the "additional vhost on port 8888". It's not clear what you mean by this. What is the extra vhost serving? If it's where your static html and php content is supposed to come from then restricting to localhost only means you're going to have to also configure an internal proxy to reach it. You can do that with Rewrite rules but that seems like an overly-complicated way to go in this usecase. Why is this vhost available to localhost only? For that matter, why are using a separate vhost... you can do this all (Plone, static files, and PHP) in just one vhost with the appropriate ProxyPass lines (or Rewrite lines if you need more flexibility).
I'd recommend rolling your configuration into a virtual host block. You can deliver static content directly form apache by rewriting a specific path. Here's an example
<VirtualHost *:80>
ServerName yoursite.com
Alias /static /var/www/some/path/
<Directory "/var/www/some/path">
Options Includes FollowSymLinks
AllowOverride All
</Directory>
# Zope rewrite.
RewriteEngine On
RewriteRule /static - [L]
RewriteRule ^/(.*) http://127.0.0.1:8080/VirtualHostBase/http/%{SERVER_NAME}:80/Plone/VirtualHostRoot/$1 [L,P]
</VirtualHost>