No difference. Last Visit time stamp lags by several hours and lag is inconsistent (at least 2-3 hours, most of the time 8-12 hours, sometimes up to 18 hours) -- so much so that I wondered if (a) Last Visit was triggered or set by the first visit of a calendar day, (b) if the difference is due to time zone differences (PDT, EDT, UTC), or posting activity. One would expect that Last Visit would be related to the _utma, _utmb and _utmc cookies -- but I see no correlation or pattern yet. As the problem occurs over multiple browsers and OS versions, all I know is that it is a server side problem.
Things to check on the server side would be:
1. Server time setting.
2. Server time zone setting.
3. vBulletin default time zone (including whether or not PHP oa_settimezone() is being used correctly).