any check engine lights?
more specs of the car would be helpful. I'm assuming your car is a GS-R, correct?
A few causes could be the following:
Not enough oil in crank case.
Low oil pressure (see note about the VTEC oil pressure switch below)
Knock Sensor failed/failing (if it's OBD2. OBD1 knock sensor does NOT effect VTEC engagement)
VTEC oil pressure switch bad (or could be that the car doesn't have enough oil pressure to engage VTEC)
VTEC solenoid is bad or needs cleaning (this is really easy to do if you need to)
Throttle position sensor is faulty or not calibrated correctly (if the TPS isn't calibrated correctly, you won't have VTEC)
Now, if you're lacking oil pressure, there could be several reasons. Here are the most common causes.
Again, not enough oil in the crank case. Check how much oil you have, first. This should be the first thing you do, always.
Oil may be old, and not have the viscosity needed to provide consistent pressure.
The oil pump may be going out and in need of replacement.
Oil pickup in the crankcase (that pulls oils from the pan through the rest of the system) may be damaged. This could have happened if perhaps the oil pan took an impact and dented it. It MAY have damaged the oil pickup. If the previous owner, or perhaps you did it (not pointing fingers, or anything. just pointing out possible scenarios for you that might help you find out the problem), replacing JUST the oil pan won't fix the problem. If the pickup was damaged, it needs to be replaced. this is one thing a LOT of people overlook when they have VTEC problems. It has been the culprit of a lot oil pressure related VTEC problems, and a lot of times, people did everything else BEFORE checking out the oil pickup.
Here are the things I'd do, in this order to diagnose.
1. Check oil level and condition. if it's really black and grimy, it's gotta go. or if extremely runny.
2. Pull the VTEC solenoid (shouldn't need to pull the whole thing, just the top half above the oil pressure switch) see if the screen is clogged up with anything. if it is, clean it up)
3. Check the calibration of the Throttle Position Sensor (google the process, it isn't hard to check. Calibrating it can be a pain, but simply checking the specs of it is really easy)
4. Check oil pan condition. If there are any noticeable dents, this is a big red flag. PULL the pan off (follow obvious oil draining process first, otherwise you're in for a MESSY treat) lol
5. If the pan is dented, get it replaced. It'll only be in the way. At the barest of minimums, try to straighten it back out (as long as the baffle isn't messed up, which it probably is if the pan has a significant dent)
6. Check to see if there is anything wrong with the oil pickup. If it's dented, bent, or otherwise flawed, replace it. Even if this isn't the immediate cause, it's really easy to replace and will only help. can't hurt.
7. If those are all in good shape, then it's time to get into the nitty gritty. Time to start disassembling a little bit. Remove all your belts, pulleys etc. to get access to the oil pump. (you'll be removing all your accessory belts, all your timing covers, so yes, a timing belt/tensioner/water pump replacement makes sense if it hasn't been done in a while) as well as removing the pulleys, and main the crank gear that drives your timing belt. once all that is out, you'll be removing your oil pump. check to see if it's working correctly. a reputable service manual will come in handy to tell you everything to look for. but, since you've already gone as far as taking it off, you might as well just put a new one on.
hope that helps. feel free to hit me up if you need any help.