How to link Cs-Cart with other Login?

I am using vbulletin 4 and a directory software that links together as well as the cs-cart. I was wondering if it might be possible to combine the logins so when my users login to either of the services they can be logged in to the other services as well. I have been trying to do this for some time. i actually tried rewriting the config.local.php and that screwed up the entire cs-cart. Luckily I was able to bring it all make by repairing my database. Anyway is there a way to do this? Also does anyone know do you need .php on any page that has a login? Thanks