I just completed a test (without x-fering attachments) of migration from MyBB 1.6.x to vBulletin 3.8.7 to Xenforo 1.1.2
I am missing 4 posts, 10 threads, 15 members, and all avatars and attachments but all-in-all I'd say this is a viable route...
Dealing with 1.8 gb of attachments won't be fun. I wonder if the phpBB option is easier/quicker...?