You need a certain amount of oil pressure for VTEC to engage. Mechanically, VTEC is driven by oil pressure. However, the electronics decide when it is engaged not the oil pressure itself. If your oil pressure was low, VTEC would not engage. I doubt that oil pressure is the problem though, since you are using an AEM, I would guess it is something in the tune or a defect in the ECU. If you have the stock ECU, I would give it a try.