Sharp brakes, can the vacuum booster be adjusted to give a more cushioned feel?

No adjustments are possible. You have a very light car that has brakes that are better than most. The soft feel is something most people want to be rid of. About the only thing you might do is replace the brake pads with some that are not as aggressive as the factory type, but that would also impact stopping distance as well. The other option would be more practice at smooth braking. That is just something that requires practice with the smaller cars. The other thing is to use the engine braking more than the brakes themselves, that is also a learned skill.

Sorry it isn't better news but there are limited options when it comes to factory brakes.