Why, it would be easy to -create- maximum reaction delay of 0.01s on -any- of these chips, it's just that even if you screw up your design really bad, and get the 4s delay, it will still get approved, instead of getting you fired.

What I meant is, while with 8081 because of typical to CPU "slower" IO you get the reaction within, say, 0.01s, and with 8052 you could get the reaction some two orders of magnitude faster, still the speed is not a reason to choose a '51 since we are far above safety value.
Really nobody will care if they get their candy 0.5s after they pressed a button or within 0.0001s.

Still 8052 is a better choice for that job, simply because the device will contain maybe 20% of the parts that would be required if you used a CPU instead, you will be probably able to change the candy pricing using a laptop with serial cable, without switching the machine off, instead of detaching a EEPROM chip and reprogramming it in external programmer (or eventually, changing contents of the boot CD of your vending machine??) and with the -design- savings, the machine would be probably some 60% cheaper too.