I have the variable set, or at least I think I do. Here is my config for the vhost.

server {
client_max_body_size 20M;
listen 1192;
# .domain.com will match both domain.com and anything.domain.com
server_name rentavault.net;

# It is best to place the root of the server block at the server level, and not the location level
# any location block path will be relative to this root.
root /home/www/rentavault_net;

# It's always good to set logs, note however you cannot turn off the error log
# setting error_log off; will simply create a file called 'off'.
access_log /home/www/rentavault_net/logs/access_log.log main;
error_log /home/www/rentavault_net/logs/error_log.log info;

# This can also go in the http { } level
index index.php index.html;

location / {
# if you're just using wordpress and don't want extra rewrites
# then replace the word @rewrites with /index.php
try_files $uri $uri/ @rewrites;
}
location ~ ^/cgi-bin/(awredir|awstats)\.pl {
gzip off;
fastcgi_pass 127.0.0.1:9000;
fastcgi_param SCRIPT_FILENAME $document_root/cgi-bin/cgi-bin.php;
fastcgi_param X_SCRIPT_FILENAME
/home/www/rentavault_net/$fastcgi_script_name;
fastcgi_param X_SCRIPT_NAME $fastcgi_script_name;
include fastcgi_params;
}

This config will not let me access the sub directory without index.php in the path of the browser. Very frustrating.