"Vapor lock" you will see nothing but air in the fuel line and it will die at any speed because vapor lock is the fuel unable to reach the engine because it evaporates before it can get there. Very obvious because the fuel line will be completely devoid of fuel (and not just the bubble of fuel in a loop in the line that is normal).
If is it a condenser problem then you will have a weak or feeble spark when it decides to give you trouble. If the spark is blue and snappy then it is not the condenser or coils or points. Could be a weak connection on the bonding strap, which should always be relocated to a intake stud rather then the upper engine mount bolt.
Could be the ignition timing which allows her to drop off at idle.
Also do a leak down test - super easy to do and will verify the containment of the engine. Throw a oring on the intake to carb union when you are done the leak down if it is successful.