A vacuum advance reacts to changes in the engine's load to advance timing. A mechanical advance will react to an increase in rpm to advance timing. A vacuum advance kicks in quicker than a mechanical advance. You'll need both to get your full distributor advance.
If it doesn't have a vacuum advance then it certainly doesn't have a mechanical advance either. Pop the distributor cap and rotor to check it for yourself. Your distributor uses the computer to control the timing advance. If you use a computer controlled HEI without the computer to advance the timing curve then your engine will only have the base timing and it'll run like crap. A good aftermarket HEI will run around $200 or you can pick up a used late-70's/early-80's HEI from a junkyard for $25-50 and rebuild it for another $50-75.