Linux Apache Rewrite Rule
 

Interesting issue here.

I have this under a vhost along with many other rewritecond's that work fine.

<code>
RewriteEngine on
rewritecond %{query_string} page_id=20 [nc]
rewriterule ^(.*)$ http://newurl.com [r=301,nc]
</code>

This doesn't seem to work after reloading Apache's config. According to documentation, this should work; any ideas?

It works indeed.
How do you test it? You should use something like: http://vhost.com/index.html?page_id=20 and see if it redirects you to http://newurl.com.