**Question about VNAV on Boeings**

Hopefully there will be a kind pro out there willing to take time to answer this question.

As I understand it, in the crudest and most general terms, VNAV plans a descent from cruise by drawing a line up from the runway to a point (T/D) on the active route where, taking account of forecast winds and other known constraints, if you close the throttles, the aircraft ought to be able to glide down in idle thrust all the way to the runway (or to 5-10 miles out on the glideslope, anyway). I understand there are Speed and Alt Intervention modes to take account of temporary unforeseen ATC speed or alt constraints. Roughly right so far?

if so, my question is, how does the computer (FMC?) which calculates the position of T/D on this basis know in advance how many track miles Approach is going to vector you round? Does it, for example, use some sort of assumption about this and, if the track miles turn out to be materially different, the crew just has to abandon VNAV and use some other more "traditional" A/P modes?

Thanks in advance.