I've had an antagonistic relationship with a vendor like this, it's awful. In my case the vendor was supposed to be a fast moving tech startup - the only thing that moved fast there was the revolving door of engineering talent coming and going.
Even worse, my boss had been convinced by their founder that he had all this pull with the company, and since the company was super cool, that made him super cool, and I dunno if you've ever tried to criticize something that has made a middle aged nerd feel cool for the first time in his life, but let's just say it was not a fruitful endeavor.
The number of things I effectively fixed for them via email, the abominations I had to construct to work around the things they refused or failed to fix...bad times.
Old comment by now, but you're a lovely engineer for that and I, for one, appreciate you lol