Your precise vesting schedule would have been in your award agreement not in the contract. You would have received that a few months after you started, and the entire stock incentive program plan summary is available on Amazon's inside HR site (it's only 11 pages, and you should have received a copy of it). If you have never read either of those, that's not lacking clear disclosure or transparency on Amazon's part; that's you not paying attention. If you did read them and didn't understand them, there are tons of people who could have explained it.

But even more fundamental than that, Amazon's standard vesting schedule is not at all secret. You can just google it. 99% of Amazon's employees are on the exact same vesting schedule.

So maybe the way to look at this is not that you lost 15% of your pay, but instead that you paid 15% of your salary to learn a valuable lesson to always read your contracts and understand precisely how and when you are being paid.