There is a Thank You module for Vbulletin. In this way, we can avoid long threads of Thank You messages.
You can thank a person by giving them positive reputation...that's the best way and that's why the reputation system was created.
Yeap, you can thank them by sending a PM or giving the good `ol green rep. I installed that thank you modification on my test board and removed it laters due to frequent upgrades/bug fixes.
Completely agree, PM is the best option for thank you messages.. However it would be nice, if we can have a module to reply post via PM. Something as discussed in this thread: http://forums.digitalpoint.com/showthread.php?t=202255 Cheers!