Thanks again wiggy.
So would I be roughly right in thinking that VNAV's fine for the first bit of your descent and approach down from cruise until you contact approach and they say "Vectors to runway XY, Z track miles to run" whereupon 9 times out of 10, you'd be as well switching VNAV off and applying the 3 x table and requesting more track miles if necessary? If that's approximately correct, then it's the answer to my question.