Although the manual states that at bug +15kts VNAV path will default to VNAV speed, its depends on each individual A/C. Some I have flown will as advertised default to VNAV speed at spot on bug +15, whilst others will cling on to the path for dear life.
With the former case, if you sellect FLCH and increase the descent speed to 'dive' below the path (approx 300'below/10kts increase in inspeed B757/B767) then resellect VNAV, as the speed reduces back to the target airspeed you will recapture the path from below.
There are many ways to skin a cat and this is only one suggestion.
Accurate descent planning is a fine art not an exact science.