I think you need something like this on the scotch domain:
RewriteCond %{HTTP_REFERER} !^
http://vodka.domain.com.*$ [NC]
RewriteRule .*\.htm $
http://vodka.domain.com/403.htm [R,L]
so that only if the referring url is vodka.domain.com will it load. Otherwise you get the 403 page redirect.