Pre-Takeoff program the Take Off page 2 with the correct values depending on NADP in use (1500 thrust reduction in any case, acceleration according to NADP [1500 or 3000], EO acceleration at 1500, 1500 being a SOP value).
After takeoff and gear up select VNAV at 1500ft and clean up according schedule, the autopilot can be used from 400ft AGL on. As soon as boeing is able to deliver the VNAV T/O able software select VNAV before takeoff, same as LNAV allready.