vakvak, are you asking if that option is a hack or an already existing feature on vBulletin?
If that is what you want to know...then that is a feature of vBulletin (it is NOT a hack).
You can set that through your Admin CP:
Log-in to your Admin CP
Go to vBulletin Options
Go to Thread Display Options (showthread)
In Show Users Browsing Threads, select Yes, Alphabetical
Then click Save
Hope that's what you wanted to know!