A Vitamix is just a "stronger than average" blender, and retains the fiber, which juicers remove.
A Vitamix is also more expensive, but can process most fruits and veggies whole, which might ruin the blades and burn the motors of other blenders. alternatively, you can use a cheaper, less powerful blender, and cut/chop up your fruits and veggies first.
Many people with digestive conditions have a problem with fiber. Takes longer for your system to digest and extract the vitamins and other nutrients, which can end up passing through instead. It can also cause gas and bloating. Juicing removes the fiber so the nutrients are assimilated into the blood stream much quicker and more efficiently--meaning your digestive system doesn't have to work as hard.
Juicing is usually preferable for those who have problems with diarrhea, as well as gaining/retaining weight, as it helps them absorb more calories in addition to the other valuable nutrients from their fruits and veggies.
The left over fiber from juicing can also be saved and used in recipes--as a wheat substitute. For example, we use the fiber to make zucchini bread and tortillas.