I've had a few people over the last few weeks mention this problem and I've done something that I think might help.

after researching the issue online I found that it happens most frequently to users that use AOL or other similar mass-dialup Internet Service Providers because of the way their proxy servers work which causes ones IP address to change frequently. The vBulletin system, for security reasons, will log people out if their IP address changes too much.

What I've done is increased the lee-way vBulletin gives to IP address changes in the hopes that it will solve this issue.

If you are still having this problem going forward please post here and let me know so I can keep digging.