The flawless Diamond is the most rare and valuable of it's kind. To have no inclusions or blemishes under a 10x microscope is extremely exquisite. At the same time, the necessity for flawless is less priority for the purposes of an Engagement Ring.
Very, Very Slightly Included are the next most valuable on the scale of clarity. These are impossible to see with the naked eye, and are even hard to see with the 10x scope.
The Very Slightly Included grades are more popular in Engagement Rings. These Diamonds will have small inclusions only visible with the 10x scope. With these grades, there are still many different 'levels' of VS1 and VS2, meaning there will be some stones with black inclusions, and some with white inclusions. Depending on the color and locations of the flaws, the Diamonds will be more or less expensive than one another.
These stones will be some of the most popular choices among Engagement Rings, because of the price, but it is very important to know the reasons for each difference in price. SI2 and SI1 stones can be thousands of dollars different, depending on the color, size, relief and location of the inclusions. If the inclusion is dark and right in the center of a stone, it should be much less expensive, and for good reason- you will be able to see it with the naked eye. On the other hand, there are some better SI2 stones then SI1, because of the location and color of the flaws.