Disable mod_proxy for subdirectory (apache)

You can just ignore the username and anything that follows when fixing up the redirect:

ProxyPassReverse /~ http://www.userdir.com/~

Since this is just a prefix substitution.

You need to do this via two independent <VirtualHost *:X> directives. Your HTTP directives go inside <VirtualHost *:80> while your HTTPS directives go inside <VirtualHost *:443>. Adjust as required if your server has multiple Address Based or Name Based virtual hosts configured. See the Apache 2 documentation for full details.