You claim that "too much toe-in will make a car's steering feel light and very vague". I think you're getting your toe-in and toe-out mixed up. Toe-in will give a pronounced self-centering characteristic that helps straight-line stability. It does, however make the steering heavier. Excessive toe-out, on the other hand, will make the car wander all over the road. Toe-out at the rear can make the car nervous under braking, toe-in will make it more stable, for all the same reasons. We play around with this on the racing cars we run, but obviously are less concerned about inbuilt stability. Indeed, we run a little toe-out on the front to improve the initial turn-in grip and run the rear at zero to reduce the frictional losses.