Lots of threads here detailing migrations from vB4 to XF 2. vBulletin is a failing platform, and is losing market share every day. XenForo is a winning platform, and taking market share from vB by the day. I also think you'll find that a vB5 migration is perhaps not as easy as it is touted to be.
It's not difficult to do the import. The XF importer works well and is well documented. It's been refined and capabilities expanded even since I did my migration just one year ago.
As far as customizing a new XF site -- that depends on what your current site looks like. There are some excellent purveyors of XF styles, to start with
@Pixel Exit and
@ThemeHouse, but many others. So you can buy an off-the-shelf style and modify it to suit your needs. Or you can engage a custom developer (or perhaps one of the established style vendors) to create or tweak something for your needs.
Being fair and honest, XF doesn't have all of the built-in "bells and whistles" that vB4 has. But XF also is not lacking functionality. It's sort of like buying a car without the rear-window defroster or auto-dimming rear-view mirror, or auto-sensing windshield wipers, vs. a car that has all of this stuff built in. With the minimalist car, you can still flip the switch for "night mode" on the rear view mirror, or turn on/adjust the windshield wipers ... it's just not automatic. None of it affects how the car runs and operates (what really counts), but all of it makes things just a bit more comfortable to own and drive. I have also heard (but have no direct experience) that vb5 was not as "full featured" as vB4 was -- I don't know if that is still true or not.
That said, you can get 95% of vB functionality on XF via free or inexpensive add-ons. I've implemented a number of add-ons for my forum that have brought it up (functionally) to the standard of what I had with vB. The XF developers have taken more of a "minimalist" attitude with bells and whistles .... they tend to focus on the substantive stuff that is super useful or very much user-requested, and executing it well. That is in opposition with vB's philosophy, which has been to pack it with features and capabilities -- a lot of which doesn't get used and results in very bloated (large) software and an increasingly creaky and fragile platform.
Read up the probably dozen threads here on vb4 migrations, and look at the XF documentation detailing the migration process. That will probably answer 99% of the questions you'd have about a migration. Good luck!