The problem is what happens after the party.
When infected, vaccinated people are around high risk immunocompromised people for whom the vaccine might be less effective or the unvaccinated they pose a risk because they shed just as much as an unvaccinated person. This is the part all people need to understand. It doesn’t matter how you got infected and there will be other circumstances besides this that will lead to breakthroughs. The same ones that led to a high number of infections as before. People working in extreme close contact, medical workers and the like.
Also, if you are engaging in extra close and personal contact, people should want to know the vaccination status and positivity status of their partner to avoid leaving the encounter with a special surprise.
Now we understand why experts were saying “you don’t need to stress test your vaccine.” Although that provided the conditions to show that vaccinated people could still spread Delta.