Hi @jaeitee given that you have an existing Invision Community installation:
1. Prepare the existing Invision Community installation by making sure it's on the latest version. Same thing with your vBulletin installation. You also want to create a test copy and a back-up copy at every major step.
2. You should make sure that your membergroups are set-up correctly for the 'combined' installation, since you will need to map those permissions during the import.
3. Run some test migrations if needed.
Some other notes:
1. You can check out this blog post that I wrote with real experiences of vBulletin clients switching to IPS. The members that I interviewed are actually all real and active members of the community. (One of them was active ... 18 min ago!)
2. In terms of community strategy, you want to communicate, communicate, communicate. Invite over your superusers in a private access test; send out blast emails, send out notifications, share screenshots, send instructions, etc. Then repeat again as you get closer to the actual migration. As Rhett mentioned above, any accounts that share the same email will be merged, so don't be afraid of inviting over members to preview and acclimate themselves to the new community.
3. This is totally up to you and how much of a hoarder you are 🙂, but you might want to think about cleaning out your vBulletin database. Delete old topics, remove outdated instructions and how-tos, revert to vanilla, etc.
4. For the migration itself, you can ask IPS to do it as a paid service, a third-party provider do it as a paid service, or yourself. For list of third-party providers (and then filter by Data Conversion): https://invisioncommunity.com/third-party/providers/