From A manufacturer's standpoint...
All questions are good to ask, however, you should have developed a business plan ahead of time. If you are considering adding a product or capability, you should know what you want to accomplish. Will this product add to our capabilities? Will it increase our potiential customer base? Will it provide us with a competitive advantage over our competition?
When interviewing potential vendors, make certain that the product fits your plan.
The capabilities of manufacturers will differ, as will the capabilities of product. Some may be stronger on one aspect, while weaker on others and vise versa.
The questions you formulate, should be related to the goals and criteria that have been set within your business plan. Specifically, the technical questions should relate to your specific needs and concerns.
With regards to PLCs...whoever you interview with (representative of the company) should know the product inside and out. He should be able to relate their products to the competition and provide you with pros and cons of their products vs. other manufacturers. With the larger companies, chances are you will meet with a manufacturer's rep (independant of the company), or possibly a regional sales manager (employed by the company)...know who you are dealing with, because the level of information you receive will be different. The factory rep, should be able to provide you with more detailed information while the mfg. rep. information will be less detailed. If you are looking for technical information, chances are most sales reps will not have the information you require...they will probably have you deal with a technical support reprensentative.
Eventhough I didn't provide you with specific questions, hopefully this will give you direction as to how to formulate your questions, based on your company's needs and goals.
God Bless,