The motor/ECU have to come together on a couple points to engage vtec.
Tempature has to be 140 degrees(I believe maybe 120)
RPMS, have to be at whatever your ECU is set to
you have to be going forward at 20mph
and oil pressure needs to be XXpsi
If for some reason your rpm signal is not working, than you will not have vtec, same with your speedo.