Where are you?
In Australia if you're told to "descend to 6000" then you do it straight away, if told "when ready descend to 6000" then you can do it when you get to your own top of descent point. Normally when under vectors you're in the terminal area and they want you to descend straight away. In the USA they use the phrase "pilot's discretion" to indicate you can descend when you want to.
If you're using VNAV and you're told to descend before the VNAV TOD then you should be able to use a VTO type function to link your present vertical position with the level you're descending to. This will start a descent straight away using VNAV but at a shallower angle so you still hit your next VNAV waypoint. If this doesn't give you at least 500 fpm (or whatever min rate of descent is required where you are) then you need to increase the descent rate and forget about VNAV for the time being until you can get back on profile.
Another option is to use the VNAV for guidance only, you may be able to get a VNAV display up on the PFD but use another descent mode to follow it, that way you can see where the VNAV thinks you should be but you can adjust the descent rate as you like to meet ATC requirements.