I'm a native Dutch speaker, and I actually haven't ever payed attention to this issue. Now it has come to my attention, I think I can confirm what others say here: That the Dutch "v" is only partly voiced. I think Kuba is right in that you must start with a bit of an "f" sound (unvoiced) then move to the English "v" sound (voiced).
I think that for making the "f" sound, I tend to press my lips tighter than for the start of the of the "v" sound, so the start of the "v" sound is not entirely like an "f".
My best guess is: Start unvoiced, with your lips a tiny bit more apart than for an "f".
I need to study it more though.