This is a weirder question, but here it goes:
During launch, a payload (let's say a satellite) undergoes intense shaking and acoustic loads. This is known from the launcher parameters and the satellite is adjusted to be able to handle these loads, undergoing environmental testing before. The payload fairing on a rocket is designed to protect the payload from some of that, besides the other thermal and atmospheric effects.
From answers to Pressure inside a launch fairing - falcon 9, we know that the environment inside a fairing is pressurized.
My question is why isn't it a vacuum instead? Wouldn't this negates most of the acoustic loads on the payload, and thus make the shaking the main concern during launch? I see that a good vacuum would be pretty hard to achieve inside a fairing, and there are lots of small problems with that, but I can't see what they could be.
I see this as a possibility for more delicate payloads, say a telescope mirror, but I don't know in that case whether the engine shaking is a bigger problem than the acoustic shaking generated, and whether that effect is as problematic as I'm thinking.
Might be too much for this question, but I'd greatly appreciate if someone could provide a reference on what's harder to design for in a launch (vibro or acoustic?).