wouldn't it hold that just as many unvaccinated were exposed and didn't catch it as well? Is that not also possible?
No, because the unvaccinated only make up a very small proportion of the U.S. population, and because we already know the % effectiveness of the vaccine from field studies where measles is/was endemic.
Additionally, the current flu vaccine is less than 30% effective for preventing the flu. At that point it could be the person's own immune system preventing it so it is completely worthless.
This isn't quite true, either. First, it's not "worthless" if it's preventing about one in three vaccinated people from getting it. Get the shot, you don't know if you'll be one of those three. Secondly, the effectiveness of flu vaccines isn't known until several months until after you should have already gotten one, so pronouncing it worthless is after-the-fact. You should have already gotten a flu vaccine, and should get one every year. Lastly, immunity to related strains confers protection that results in an easier course of illness (shorter and less severe) among the vaccinated. A body vaccinated with influenza A, H3N2-Texas strain will do better than one that hasn't been vaccinated against the currently circulating, related influenza A, H3N2-Switzerland strain.
Besides, if you have a 50% chance of catching something after being vaccinated, how is that better than being unvaccinated?
Wrong. This is exactly what I was trying to point out! It's not "50% chance of catching something" for the vaccinated person - it's still 2% to 10%. 50% of people infected were not vaccinated. Not 50% of those vaccinated. 50% of those infected. Not 50% of vaccinated. If you don't get the difference at this point, I suggest contemplating the denominator, taking a class in epidemiology, or giving up and taking the word of someone who gets it.
I absolutely agree that vaccines are less than 100% effective - indisputable fact - but in the case of measles not "far from" 100% effective. And as I have been saying in this thread all along: the imperfection of vaccines makes it all the more important for EVERYONE to get vaccinated to maintain herd immunity.