correct me if I am wrong but there are 3 things that control vtec.

1.oil pressure
2.engine temp
3.engine speed

so if the car is warm and has oil and you have done nothing to change the ecu vtec crossover then your car is the rare 2nd through 5th gear vtec(j/k). It could be your temp sensor. If it doesn't work then your car won't go into vtec. Check your vtec solenoid.....aka clean it out if the screen is clogged. Make sure you have oil so that you are making enough oil pressure. Also....but not likely.....your oil pump is going out and maybe not making enough oil pressure to switch vtec on.....just my 2 cents, im not a mechanic but have had this problem on many hondas.....hence why I have the car in my sig.....hope this helps