That depends on the vendor. Small vendors know that they can't do everything and are happy they are part of the pie. Medium sized often dream of getting big and so if they think they can by taking a large slice they will.

It also depends on how your relationship is structured and what you demand. I work for a very large company, but some of our customers won't even look at us until we pass a third party interoperability certification, and thus getting that certification becomes critical to us even though most customers don't care. Once we are certified interoperability issues are rare (they happen all the time because of the sear number of customers, but most of the time things just work because everyone is following the standard). The standard and certification has been refined over a couple decades now and so most of the things that can go wrong either are either updated in the standard and certification test; or they are at least tribal knowledge of "don't do that it won't work"