The best way to make sure that vendors keep their promises is by demonstrating that working with you will not become a zero-sum game. In other words, do not turn the relationship into a contest where someone wins and someone loses.
Make it a win-win situation in which there are no losers, only winners. Then everyone will be happy and everyone will benefit. Ideally, the relationship will continue for many years, generating profit and gain on both sides of the buyer-seller equation.
In my experience, all such happy relationships begin with strong contracts. Players on both sides need to know exactly what they're getting into and what is expected of them. Ideally, contracts should function like the rulebooks used in competitive sports. In sports, there are always disagreements and disputes. The rulebook provides a guide for resolving issues and getting on with the game.
A significant difference between you and the manager of a sports team is that in your role as the CIO, you will have to serve double duty—sometimes you will be a manager and sometimes you will be a referee. So you better make sure that your rulebook covers all the possible contingencies. If it ...