"The next question is: using an external soundcard the VMS4 led meter are inactive. Is there a way to re-activate them or I'm forced to see the traktor meter in my laptop?"
if you are not routing audio to the VMS4 the VU meter is not processing audio, right? if you are using an other sound card for what ever X reason than there is no VU meter action on the VMS4. Unless you run the other sound card to the VMS4 so you can see the meter in action.