Imagine that you only have 1 car. It gets 2 miles per gallon.
How many miles a week will you drive it? Will you drive it to the store that's 0.5 miles away or walk to the store? Will you see your friends every night by driving to their places?
Now imagine someone gives you a Prius which gets 50 MPG.
How many miles a week will you drive it? Probably more than however many miles you drove the last car, huh?
Same with VaporFlys--you don't feel as tired/your muscles aren't as beat up...but will the additional training you do because you don't feel tired negate the benefits of the shoe?