Fix bug - All user's config_dir set to same global path. 

Before this fix when using `vim::userconfig` the user's config_dir was set to `$vim::config_dir`. So all user `.vim` directories were set to the same location (by default `/etc/vim`). You could only change the main `$vim::config_dir` param (with any nextgen param_lookup data source) which still didn't allow multiple users to have their own discrete `.vim` directory.