I know this is a very vague question, but...

what vehicle parameters affect spark advance the most and why?

 

Vehicle as in engine parameters, or attributes of the car in general?

 

Do you mean when is more or less advance required?

 

yeah, what parameters change why you would increase or decrease ignition timing.

 

In general the greater the load on the engine (less vacuum/more throttle applied) the less advance must be. The higher the rpm, the greater the advance must be.

These two rules must be applied simultaneously.

One example is, at 2000 rpm and 30kpa manifold pressure (no throttle) timing would be around 25 degrees (or more depending on application). But at 2000 rpm and 90kpa manifold pressure (almost full throttle) timing must be decreased to around <15 degrees.

You should buy a book called designing and tuning high performance fuel injection systems by Greg Banish. There is all you need to know about tuning in there.