If a candidate is not prepared to be president, they are not prepared to be vice president, either.

But I don't fully agree with your description. I think there are other considerations as well. First, you narrow down the pool to who could be a good President, since that is a requirement of the job - that basically is the job description.

But then, you want to look at who you (assuming you are aiming to be President and thus making this choice) could work well with. I think it's useful for the President to get along well with the Vice President. It's not required, but I think it's better if they do.

You can then also look at who will help you get elected. As long as the person is qualified, I'm okay with some strategizing for political reasons with the pick.

It's just that you have to pick someone qualified. Putting anything ahead of that is reckless and our country deserves better.

Pragmatically, candidates will look at how many votes the VP can get for them, but that's not really a job qualification as far as the voters are concerned. (I was really thinking about this claim from the point of view of the voters, though from the description I can see that it may not have come across that way.)

Perhaps the claim is oversimplifying a bit; I agree that having a VP that can work well with the president is important. (Conversely, having a president that can work well with the VP is also important, so one could reasonably claim that those qualifications are, if not identical then at least symmetric.) Also, a VP as president would have a slightly different role as president: he or she would be taking over in a crisis situation. Therefore, it makes sense to have very stable people as VP.

That's true... not necessarily a crisis exactly... the President could die during a fairly calm time period, but certainly without the usual transitioning and preparedness for it. But there is a higher probability if the VP becomes P during that term that it will be during at least some degree of turmoil.