Which version of phpvms you are using I do not know, but user fields are working fine ...
For phpvms v7 dev builds, go to admin -> users -> profile fields and add what you want there. Example;
Name : VATSIM ( No spaces here, it will generate errors )
Description : VATSIM ID or VATSIM CID
Required : Optional
Show On Registration : Optional
Private : Optional
Active : TRUE
Below is from my live site;
Displaying it depends on the theme you use, older versions of default theme had a problem regarding their display... Fixed a while ago, also I am not sure about beta4 if you are still on it.