There's a reason that vacuum tubes are tube-shaped. It's so they can withstand the atmospheric pressure forces when the air is sucked out. The curved shape of tubes and globes actually make those structures stronger as the external pressure increases.
Your plate won't have any of that going for it. If you use a high vacuum, each side of your your plate will feel a force as high as 121214.7=2116.8 pounds. I think 1 ton per side would be enough to break the glass, unless you use something thick like a telescope mirror blank or a porthole glass.
Try a smaller plate first. You don't have to use silicone rubber, and you probably won't need to glue it, either. The good news is that as you lower the internal pressure, the increased force will squeeze the rubber more, actually improving the seal. Use a stiff grade of rubber. If the rubber is too soft, it may squeeze down too much leaving you no gap.
Good luck with your project!