Rep Power: 0

I've had my car for maybe 5-6 months now, and ever since I bought it, the engine has been spiraling downward into oblivion since day one.

For starters, the VTEC has recently packed its bags and left my engine, and the car has no power whatsoever. The gas mileage has suffered tremendously, I was getting about 34 mpg avg at the start, now I'm getting about 25 avg. The car had no upstream O2 when I bought it (it's got one now), and I was driving without one for those 5 months, so who knows what damage that has caused to the cat, which is now throwing a CEL cat deficiency code. The car idles/shudders horribly at 300 rpms with the A/C compressor on, and at about 500 rpms with it off, although average idle speed should be around 750 +/- 50 revs, am I correct? I'm not sure when the spark plugs and cap/rotor were replaced, or if the fuel filter has ever been replaced, so I'm probably going to start there, but I doubt that will fix the VTEC absence.

Any help on these issues would greatly be appreciated, I'd hate to have to drop a new motor into a Civic with only 100,6xx miles on it!