You posted in the wrong forum, you will get the best help by using the correct forum, but I can see you are a new member - so no worries. In the future try to Google vbulletin.org or used the search feature here as there are several 'mods - modifications' here on vbulletin.org that might fit your needs.
Here are two that are popular.
[AJAX] Post Thank You Hack
https://vborg.vbsupport.ru/showthread.php?t=165673
[DBTech] Advanced Post Thanks / Like v3 (vB4) [AJAX]
https://vborg.vbsupport.ru/showthread.php?t=243510
I hope that helps, have fun.