It should be similar to the Kunena3 import. You’re probably better off installing a Development environment than trying to it in a container.
The importers that get developed are the ones that Discourse’s customers need.
You get the script for free. For $100-200 bucks you could get someone to import your data for you and give you a backup file that you could restore on your server. It’s likely that there’s something funky with your database, or the script doesn’t import everything you want, though. (I don’t know about the vBulletin script, but the Kunena3 script doesn’t deal with attachments, for example.) When I do an import, I clean up the script a bit, and if when I get time, I submit those improvements back to the community. (And Real Soon Now, I’ll submit code for providing links to attachments in Kunena3. Sometime later, someone will fix it so that those attachments get squirted into Discourse rather than linked to where Kunena is storing them.)