These are not vendor specific. Where there are a number of industry associations in the same sub-field, certifications may overlap. The various Webmaster certifications are a good example. The vendors and the industry associations are the only recognized certifying authorities, and they insure a constant feedback loop from the instructional and testing platforms. To be sure, providing these opportunities to learn also creates a skilled labor pool.
Some vendors function as schools themselves.
They offer traditional classroom instruction at many locations around the world. They certify instructors, rent facilities, and charge tuition for the course work. Most vendors, however, link with commercial and training companies. Despite review and authorization, the quality of training may vary widely. Some training partners offer their own certifications in the same fields as those offered by the vendors, but in a more generic, vendor-neutral form.