With the SET lastactivity part commented out, I did an export of location and lastactivity right after the update query:
5 seconds later:
Seems that vBulletin is internally handling lastactivity.
Furthermore, $julian is empty for some reason:
Content visible to verified customers only.
This might be a server issue, as time_nanosleep doesn't work in PHP, and I'm not on a windows box.
In any case, I am going to accept my fix. Let me know if anything is not kosher here.