I am not sure the original question was fully answered. the VEC2 is a "piggyback" unit that alters the fuel injector and spark timing events as a function primarily of rpm and manifold pressure. It intercepts the factory signals and outputs new signals. It is most useful at wide open throttle to be sure things are set a max power. At part throttle, it lets the factory ECU do all the complicated and important stuff for driveability, such as closed loop fuel control, idle, throttle tip-in response, and so on. The Factory ECU stays in place and the car will pass emission tests, including those that require connecting to the car's diagnostic port.
VEC2 is user programmable, can data log (including extra channels if you add WBO2), and can grow with additional mods like headers, forced induction. It even controls water injection on S/C cars. Sean Roe's customer support is second to none - he has called me on Sunday evening to return a call with questions earlier that same day!
But everyone is right - you must tune with the VEC2, you will get nothing unless you want to work with it. Plus the VEC2 is so flexible you can set it to damage the car with too little fuel or too much timing. A careful tune is essential.
I don't like the more "universal" modules - no flexibility, data logging, etc. I'd have to say the VEC2 is worth more than twice a fixed and non-programmable module. But I don't know much about them and maybe they can do more. I doubt the effectiveness of anything that you don't fine-tune for your particular car.
The AEM is a stand-alone ECU that I understand will need TONS more tuning than a VEC2. The upside is that it can do amazing things like traction control, knock sensing, WBO2 feedback and the like. Downside is that every aspect of driveability, such as idle, acceleration maps, etc need to be programmed into the AEM. Maybe there are readily-available base maps and good customer support, but I would expect the AEM install to be a significant project.
I want to say an AEM is over $2.5K, and IMO is overkill for a basically modified car. Unless you have a heavily modified engine or forced induction, I would not mess with the AEM.
Finally, the AEM is not going to pass emissions, as it is not OBDII and can't be interrogated by a standard inspection shop.
I suggest headers first - I understand the basic car responds well to them and filters and cat-back won't get all that much from a tune, IMO. Headers and a VEC2, perhaps?
