Deleting thread
 

lets say you have a URL indexed in google and you physically delete that thread while using vbseo and vbulletin 3.8.x. will google deindex that URL? or will they keep it indexed and just redirect traffic to your homepage?

when i tested deleting a thread and going to URL that no longer existed it redirected me to my sites homepage

in terms of google pagerank, if people linked back to the site with this thread which is now deleted, what is the impact if the link no longer exists but it gets redirected to website homepage?

After a while, I believe they remove the thread from their indexing. I would suggest that you read up on this at google - https://support.google.com/customsearch/

They won't do the redirect, you need to, and Lynne is correct eventually they will drop it from indexing.