A Vitamix blends up fruits and veggies into smoothies, keeping the fiber (a juicer separates juice and fiber).
A Vitamix can also grind nuts or grains, blend up nuts into nut butters or substitutes for dairy ('sour cream', 'mayonaise', etc.), make juice (if you strain the blended up ingredients through a nut-milk bag), make nut milk (blend up nuts and water and strain through a nut-milk bag), blend things into a creamy consistency, make frozen treats (just by blending frozen fruits or juice and ice).
A juicer makes juice. The non-centrifugal juicers can also make nut butters and applesauce and doughs by running the ingredients through using a special blank.
I think both smoothies/green drinks and juice are useful, but consider juices more as nutritional supplements to be taken in limited quantities, while green drinks are actual foods. And since you can make juice with a vitamix (by straining), if you want to get only one, I'd go with the Vitamix (I use my Vitamix at least once a day, but my juicer very rarely, though I intend to use it more often). It really depends on what you want to do: juice only, or many other things?
