Brides and Grooms tinker with the vows a lot these days. Promises from the old formulae get left out and others added in. Some of the new additions are very nice, quite moving. I do sometimes wonder whether some things were left out intentionally, or just didn't make that couple's top ten list when they were composing them. But in Christian weddings, at least - we don't see many others - two vows seem to always make it in: faithfulness, and "in sickness, in health." Notice that these are highly measurable, compared to the vaguer sentiments more popular now.
They are clearly important, and this is obvious even to young people who don't have a lot of experience in the world, and who grew up in a culture where these promises are not kept. We might suspect that these are so crucial to the definition of marriage that we cannot even loosely consider it without including them.
With life extension becoming what it is, the 21 year-olds I watched today may be making a hundred-year vow.