How well do you interact with vendors? What would former vendors say about how well you worked together?
A question about how you manage vendors or suppliers is one that you should be fully prepared to answer. Practically every interviewer will ask it in some form or another because it's a critically important part of the position and cuts to the core question of whether or not you're sufficiently qualified and ready to step into the role. Whether you're experienced as a meeting, convention or event planner or not, answer with confidence and with enough advance thought that the interviewer immediately sees you as being capable to do the work. Bring your portfolio with you to the interview and have it handy as you're interviewing, but in this instance you can single out things like references, vendor testimonials or letters of recommendation, awards or copies of complimentary emails. Visuals are a great way to prove what you're talking about. They basically do the bragging for you.