They are convinced of the vision and have an unwavering belief that they will achieve this vision, but there are many paths one can take. So to act rationally when encountering a failed attempt, given their belief, is to learn and try something else. :)

You can have tactical failures that don't invalidate the vision.

Your quote is from the section where they are explaining their first attempt at an auction house and how it failed but how their current system of third party retailers is implementing a similar idea in a successful way.

They kept the same vision/goal but they learned from how their first attempt failed and iterated on it until it succeeded.

How so? You try again when you fail like that Thomas Edison quote: "I have not failed. I've just found 10,000 ways that won't work."

There's another part of the quote that was not included that relates to being flexible on tactics (...while being stubborn on vision). So the "why" and "for whom" stay constant, but the "what" and "how" may change as you learn.

He's talking about failures related to individual steps on the way to the greater vision of what Amazon is supposed to represent as a value proposition to customers.

A single failure on a step, such as the Fire Phone for example, doesn't imply a failure of the overall vision ("Our vision is to be earth's most customer centric company; to build a place where people can come to find and discover anything they might want to buy online.").

Usually vision is paired with the concept of execution. The vision was to build a business out of being a marketplace where third parties can sell goods, they failed twice on the execution.

Now they know exactly what they are doing wrong?

$99 Prime, for example