HHKB is much easier to use on a Unixy computer than a Windowsy one, because Unix uses Ctrl sequences for cursor motion and Windows does not.
As the HHKB does not have dedicated arrow keys, home, pg up etc., transitioning to it on a Windows computer could take much longer. I tried for a day, and ended up giving up. I use function keys, arrow keys, and other cursor navigation keys far too much.
So I took my HHKB home and started using it on my Mac. Works like a treat. I have since learned the arrow keys, and that's about all I use on the Mac. Most other navigation I do with Ctrl sequences (^a beginning of line, ^e end of line and so on).
Using vim you is mostly Ctrl sequences and hjkl for navigation - piece of cake with HHKB layout.
But your biggest problem might be moving away from a UK (ISO?) layout. HHKB is closest to ANSI, except backspace is where it should be just above Return, and Ctrl is between left shift and Tab, again where it should be. Once you get used to those two key positions, you will \\\\\\ all the time when you try to backspace on a conventional keyboard (maybe, I do on ANSI, not sure about ISO).