Where does it take them? You'll need to see what the page source looks like for them in order to figure out what the problem is. Do you have a totally default skin available for them on the site to use? Set that up, have them switch to it, and then see if the button works for them. If not, as I said, you need to see what the page source looks like to see what is going on.
To create a totally default skin, you would create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save