Robbie is right about the technical differences between tremolo and
vibrato. The Mills tremolo seems to be mostly frequency undulated,
whereas pipe tremolo is both. About the only way you can make pipes
sing straight vibrato is if they were in a cabinet that was quickly
closed and opened to the air at the vibrato rate.
But as far as I can tell, the ear is satisfied with whatever degree of
either rate happens to be there at the time. I would suggest that if
Jeroen has a good RealPlayer and speakers on his computer that he go
to my web site, http://player-care.com/cb/ On the second page there
is a good example of a Mills tremolo playing a sweet ballad, and he can
see if that is the sound he wants. We really shake our tail for you!
By the way, The problems encountered in playing a violin are ordinary.
The problems in playing a violin well are legendary. Good luck.