1) Go to vBulletin Style Manager.
2) Under Postbit Templates section find postbit or postbit_legacyremplate.
3) Generate Like button using https://likebtn.com/en/like-button-generator
4) Use the following expression as button name: post_{vb:raw post.postid}
5) Paste generated code into template after <!-- / attachments --> tag.

Read more: https://likebtn.com/en/vbulletin-like-button