@Pat What an interesting experience! The venom from Vespula contains multiple components that induce pain (using different mechanisms). I am so sorry to say that it seems more probable that they weren’t injecting venom than that you are immune to these different mechanisms. It’s not unusual for a single insect to inject no/little venom but interesting that multiple individuals were not injecting much venom when defending a colony. If you do get stung again I’d love to hear how it goes!