There's Dutch data on the motivations behind passports, the difference between (vaccination only) and (vaccination or test) - the difference between 2g and 3g in Dutch terms as recovered is the same as vaccination for them is here:
www.dutchnews.nl/news/2021/11/the-pros-and-cons-of-2g-versus-3g-what-the-experts-say/ is a popular write up.
The spread in nightclubs is from vaccinated (as they're not tested on going in, so could have it), it's the higher risk of the unvaccinated catching it (and their severity) that is the justification for excluding them.
Imagine a country with 1% covid prevalence among the unvaccinated, and because the vaccine was 90% at preventing infection, 0.1% of he vaccinated.
So if 1000 people at a venue, 900 are vaccinated, 100 test and get a negative test. The test is only 80% effective, so there's 0.2 of a person there who's positive and unvaccinated, and 0.9 vaccinated and positive - so even with vaccine efficacy way above what we have today, you're still much more likely to have a vaccinated positive person there than an unvaccinated positive (this is the exact same argument about why hospitals have more vaccinated)
So now, what happens at the venue, imagine the positive person infects everyone they meet, other than the vaccine produces that same 90% reduction. All 100 (or 99) unvaccinated people are now infected, but the vaccine has prevented 90% of the vaccinated from getting infected, so it's only 90 of those, and of course they'll be less seriously ill from the vaccine.
Of course, the real motivation for vaccine passports is coercion on getting a vaccine, but if you are going to have a passport, it's the unvaccinated (and unrecovered) that you want to keep out because they are more likely to be infected.
Vaccine breakthrough of course changes all that, because now the numbers who can be infected in the vaccinated group is higher so you don't get the benefits, there you may well be getting more protection from testing everyone and trying to exclude the case entirely.