Accessing Variables from Plugin in Templates

I’ve written a small plugin that pulls a vBulletin cookie’s value, checks the userid with the database and pulls various other info from the vBulletin user database. I want to use this information to have the whole “Welcome back, username!” , etc. style messages on my wordpress pages, but I can’t seem to access the variables I set in the plugin from my templates.

I have the plugin running at the init action, and the templates I’ll need it in (if that matters) are header and header_home. How can I get these variables set where they’re accessible, or perhaps just manually call that function from the templates?