I am also having this problem. I’ve got a single WordPress install, reverse proxied behind Ubuntu 10.04 server, with a single WP database on a separate server, with a similar configuration to the OP, and I’m seeing the same behaviour.
What if you installed WordPress into the /blog/ subdirectory of your blog vhost, instead of trying to remap the directory?
This would probably work, but it’s a kludge. When configured correctly, reverse proxying works for the most part, but there’s a few pages in wp-admin that cause a redirect to the root page, which is a bug.
