If you put in a switch, you shouldn't get a CEL, but it should run like ass. VTEC engagement switches to a different (second) set of maps for fuel, etc., and if you use a switch, you'll still be running on the non-vtec maps, parameters, etc., so you'll probably go quite lean quite fast. You can tell the VTEC solenoid to engage with a switch, it doesn't care, but who knows what the ECU will do to try to compensate. All that intake air may make some funny corrections from the ECU..
The MAP sensor doesn't factor into the decision to engage VTEC or not, only RPM, throttle position, and oil pressure; the ECU just uses the MAP sensor like normal.. A vtec controller is just an RPM activated switch with fuel/spark maps built in to correct when vtec is engaged, the ECU doesn't care. This is why a non-vtec ECU can be used with a vtec controller (on a vtec engine), and the ECU doesn't care. It doesn't know you're engaging vtec when you're not supposed to, because it doesn't know what vtec is..