Well, I'm sure that you'll be building a shiny new Server 2019 machine for the shiny new vbulletin, so you'll have a shiny new version of IIS with URL Rewrite...
URL Rewrite runs within IIS and uses regex to dynamically change the request, so if a link on the forum points to
http://something IIS will accept the request, rewrite it to
https://something, and vbulletin ought not to notice the difference.
Probably...