Be sure the vendor you choose can deliver what they say they will on time. This can be difficult to assess. To get your business some vendors will promise the moon. Ask the vendor to offer proof that they'll actually deliver on their promises. Quality of product or service.
Quality is critical to assess when interviewing prospective vendors.
You know you shouldn't judge a book by its cover, but most of us do. Slick talk and fancy materials can mask weak content. Ask to see sample training materials. If possible, arrange to observe the speaker deliver a program, or at least talk with someone who has seen him or her speak. Warning flags abound when selecting vendors.