I am not suggesting that you should automatically trust your IT vendor, but you and the vendor should work together to build trust. It is easier to commit to large projects and long-term contracts if you have a high level of confidence in your vendor. At General Systems Corporation we actively work to build trust with out clients in several ways.
We start almost all engagements with the smallest possible implementation which adds value. This might be a proof-of-concept or a minimally viable product (MVP) but the idea is to work under a short-term contract and focus on one important feature set. These small engagements limit the risk that the client is facing, but also provide value and test GSC capabilities throughout the development stack (graphic design, front-end, middle tier, backend, and even release processes are all proven).
Trust but Verify
Make sure you keep your IT vendor honest through demo meetings and testing. A demo meeting is when your IT vendor shows you what they have built, and testing is when you test it out without interference from your vendor. Testing is separate from demoing, and testing is far more important. Make sure to properly test in order to verify that what is being delivered actually meets your expectations.
Iterate and Maintain a Dialogue
If you’re not meeting on a regular basis with your vendor (every week is usually appropriate) then small issues can grow into big ones. If you meet on a regular basis you can answer any questions (to avoid assumptions), view the working demo (and immediately correct any missteps), and discuss any changes (as you iterate and learn from testing and early users).