Is this Vivalo worth importing...?

it is a lots but then currently the Japanese are the best at building steel track frame.

you'll have to think about whether you really want a japanese keirin frame, or a custom-build frame (£500 is enough for that) that'll fit you perfectly.

Hmm, and what do you base this bold statement on?

I think that is a lot of money for that frame. £600 gets you a custom Mercian, and less than that gets you a custom Bob Jackson... Or go to BLB/TDV like you said. Leave the over priced japanese stuff alone.