Hi @Islander welcome to Discourse meta, I’ll try to answer your questions
It’s not easy per se but it is a very straightforward process given that You’re comfortable with SSH, Databases and Docker.
Discourse has Supported for Nested categories one level deep. Topics can be created in the parent or sub category depending upon the permissions of the category.
There is a 3rd Party plugin known as Babble, this can be helpful for You.
Not natively supported in Discourse but You are free to make a plugin to do that.
It depends. I’ve done several vBulletin imports. It’s surprising how different they have all been. It may be that the script will work “out of the box” or it might need customization, depending on how perfect you need things to be, what Add-ons you have used, and any quirks your community might have in how they have used it. Imports pay my mortgage, you can check out Redirecting… which outlines issues you’ll need to consider whether you do it yourself or get someone to do it for you.