To answer the general question here -- it is, sometimes, okay to ask a couple questions at once if those questions are so tightly related that they can't really be separated. Your question is actually a decent example of that, because although you have written it as two questions, it's basically just one -- "Does the surface of a lens need to be a perfect sphere segment to operate as a concave lens" or something to that effect.
All the other details and questions are kind of ancillary to that. Once you understand what the shape needs to be for a lens to work as expected, you can answer for yourself whether the lens you proposed would work. Or, at the very least, you could ask a follow on question about what shape (or how to derive/determine the shape) a surface takes under suction or something to that effect.
From your comments, you are concerned because your question has (at this time) 3 close votes. Two of those votes are because the question appears to be about engineering. One is because somebody thought it was unclear.
It takes 5 votes to close something, and so until then it may not be something to worry about. You got some answers, it wasn't downvoted, so it could just be that a few people have an opinion but not enough to close it.
However, I can see why some feel it meets our definition of an engineering question. Particularly because you want to know if the approach you've chosen (vacuum pressure on plastic sheets) would work.
This can easily be modified so it is less about how you create the shape and more about A) what shapes are required, and B) a second question eventually about how to derive/determine/something the shape of an elastic surface under pressure forces.
Without knowing the final form of what B) might look like, I can't definitively say what would or wouldn't be on topic. But trying to determine the leading-order terms to balance external forces and internal stresses in a material could be on-topic if it is written correctly and doesn't run afoul of our policy on homework and exercise type questions.