Setting realistic goals refers to goals and expectations for you, the customer, as well as for the vendor/supplier. Try not to cater to every need of a vendor - rather, identify a few items you know you can address. Be realistic. Do not fall short of stated expectations.
For example, when a new laboratory information system middleware is coming on board, the laboratory may want to look ahead at the future staffing calendar to see the availability of technologists and supervisors for a specific time to perform the verification checks of the new middleware. However, the vendor may be in a hurry to schedule the installation and verification checks to their schedule. Set realistic goals, and communicate with the vendor, but do not cater to the vendor unless all needs are being met equally. Also, if you have promised to perform the verification checks during a specific time frame, do not fall short of expectations. Do not reschedule or delay the process, otherwise, the vendor will lose trust and respect in your abilities.