It depends on what you're trying to achieve. But in most cases, 3D printing and vacuum forming should be considered complementary technologies. 3D printers can print complex shapes, but can only do so slowly. Vacuum forming on the other hand shapes plastic very rapidly, but it requires a mold. A powerful desktop manufacturing workflow could begin with printing a mold on a 3D printer and then loading that mold to a vacuum former for mass production.
If you intend to model shapes by hand rather than on the computer, then a Vaquform™ would be more appropriate. If you plan to make highly detailed parts in very limited quantity, then 3D printing might be best. To most shop or maker space setups though a vacuum former is a natural upgrade in capability.