XF 1.4 Importing vBulletin

I have already used the vBulletin importer on one of my licenses. I have since been developing the website on a hidden domain. The site I am porting over to Xenforo was heavily modified and contained thousands of custom templates. Those templates have since (85% so far) been ported over to Xenforos system using widgets and pages. I am trying to figure out my best approach for the final import of my actual website.

What I am wondering is if I can import my vBulletin website over the import I already did. I have 500k+ posts and am not sure if this would be an issue or not.

I was thinking that I could install a clean instance of Xenforo and then download the modifications I have made and do them on the clean Xenforo, and then import my vBulletin if I have to. This will be a lot of work so I would like to avoid doing this if I can.

Is there another option I am missing? I want the import to go as smooth as possible (which it did on the first import) and am trying to stay ahead by getting advice now.