Wonder, what is the max flap operation speed for your variant of NG? Ours is 250kias for flaps 1 and 5, which includes slats to mid and kruger flaps extended, a configuration that stays valid for all approved take off flap settings (1 through 25). Therefore we do not have a leading devices max speed below 250kts on our plackard or in our FCOM (except alternate flap extension), however the non-normal checklist tells us to remain 230kts or less if more than one leding edge device is not retracted and all trailing flaps are up, with only one leading edge device it's 300kts, those speed limits are there to prevent increased stress on the flaps due to longer than normal operation at those speeds and it is sufficient to decrease from 250 to 230 during the non-normal checklist, which VNAV wouldn't do anyway as it stays at max 230kts as long as any high lift device is extended.
The point of VNAV take-off is that it is boeings preferred method of operation because it offers (their words, not mine) increased speed protection both for structural and SID limits since FMC-Version 10.8, however since that was pulled back due to several bugs we cannot actually do real VNAV take-offs yet, and therefore the 1500' VNAV operation is boeing's preparation for the coming FMC-Version.