[Zope] grokking _vh_ in VirtualHostMonster

[seb bacon]

I've just spent far too long wrestling with the VHM, but I've finally
sorted it.  However, I don't understand exactly what was going on, and
I'd love an explanation.

I was trying to get requests for

 http://www.foo.com/Bar/rest/of/url

to look in

 http://localhost:8080/demo/Bar/SomeFolder/rest/of/url

and I was using

  RewriteRule ^/([^/]+)/?(.*)	http://localhost:9080/VirtualHostBase/http/www.foo.com:80/demo/$1/SomeFolder/VirtualHostRoot/$1/$2 [P]

to do this.  It worked, but absolute_url was resolving to
http://www.foo.com/rest/of/url.  In order to fix absolute_url, I found
after much hair-tearing that the following worked:

  RewriteRule ^/([^/]+)/?(.*)	http://localhost:9080/VirtualHostBase/http/www.foo.com:80/demo/$1/SomeFolder/VirtualHostRoot/_vh_$1/$2 [P]

Why did the second version (with _vh_) work, but the first not?

Cheers,

seb