Hi,

I've tested a while ago only to guest users.
The way i did it (I'm not using anymore) was creating a plugin in global start with a similar code:

vbsetcookie('usercache', $vbulletin->userinfo['userid'], permanent);

Now you should have a new cookie name bbusercache to play with proxy_cache_key like indicated before.

Note that forums are very dynamic and didn't made any tests.