The vaccine is going to be deployed to communities at risk first, you can't test vulnerable populations via volunteer challenge trials.

That's circular. If the challenge trial is successful, start deploying it to the groups tested using that method and then shift to the at risk groups as data justifies it.

It won't hurt the at risk groups to start up high volume manufacturing earlier (it will likely help them), and there is a benefit to everyone each time anyone is vaccinated (the reduction in transmission efficiency).

Or, do the standard vaccine safety and efficacy testing, deploy the vaccine to the people who need it most first, and save more lives.

Optimizing deployment of the virus to bored 20 year olds may be popular among bored 20 year olds, but it isn't the best public health strategy.

There is no reason to expect a challenge trial would slow down the standard branch of the trial. If it gave an earlier result, the differences would be good ones! The production would have worked through initial scaling and a bunch of people would be vaccinated.

Do you think the vaccines being distributed right now were manufactured more than a few weeks ago or something?

Maybe, or you developed and scaled up production of a vaccine that only worked on young healthy adults who don't need it that much, and kills or maims everyone else. Wasting a ton of time, resources, and good will that could have been better spent saving lives instead of getting 20 year olds back to bars.

Remember it wasn't a given that mRNA vaccines were going to work, we were lucky.

It's hindsight, but none of the SARS-CoV-2 vaccines in use have that problem, nor do several more that are likely to be approved for emergency use.

What proportion of approved vaccines in general have such radically different responses in different age groups or demographics?