1. Value only matters if you sell. Chances are you won't recover the added cost of an aftermarket A/C system if you do.
2. Check into the controls first; since Vintage Air was made for non-A/C cars, the controller mounts differently and your stock A/C heater control will be an orphan, along with ductwork, cables, dash vents, etc.
3. For what you save on not buying an original A/C system you could probably buy a Vintage Air and save the space, or conversely for what you'd spend on a Vintage Air you could rebuild the factory A/C.
4. Factory A/C is excellent, if charged with R-12 Freon as designed. Use of lower efficiency refrigerants will decrease the performance of the cooling system. I often run the A/C with the convertible top down during 90+ degree August cruises, and it does keep me cool.
5. With the factory A/C you will cuss every time you need to work on the right bank, but remember you have a rare 'bird with factory A/C, and that's the price you pay for having a highly optioned car.

These are all my opinions, and I'm sticking to them. :p