You need to be thinking statistics.
1. You get ill if your body is infected with say 10,000,000 virus particles and your immune system can only neutralise 99,999,999 of them before your cells make another 10,000,000. You get better if your body can destroy virus particles faster than they can be made. If its the reverse, you die. Think something like Spitfire production vs ME109 to get the idea.
2. What a vaccine does is give your immune system a head start in battle#1.
3. A pandemic is a similar statistical exercise you need R0 to go to less than one to beat it. Anything that reduces the infection rate helps, vaccines, lockdowns etc.
Which brings up JRFSp's comment. It doesn't matter if a vaccine is only "90%" protective o an individual. What matters is getting R0 less than one. The vaccine doesnt have to be 100 percent effective to end the pandemic.