This can be resolved with:
DocumentRoot /var/www/myblog
ProxyPassMatch ^/(.*.php(/.*)?)$ fcgi://127.0.0.1:9000/var/www/myblog/$1
ProxyPassMatch ^(?!\/wp\-admin\/)([^\.]+)$ fcgi://127.0.0.1:9000/var/www/myblog/index.php
The first rule will serve the assets (images, css, etc)
The second rule match all .php requests (for no permalinks, like wp-admin/ urls, etc) with the correct .php file (using hhvm)
The third rule will match with all requests with no extension (aka permalink) (image.jpg, file.css, etc) (using hhvm)
Note: you will exclude all other no-extension files that you want in the ProxyPassMatch, or put all them in other subdirectory and exclude it.