So, my code broke because a library that was provided by the go developers used some unstable feature I had no idea about and had no real way of knowing short of checking a dependency three levels below my project? And to top that, that library was certainly subject to API breakage, the entire reason I vendored it in the first place.

It doesn't seem reasonable to me. When I developed my project, vendoring /x/ projects was recommended exactly due to the breakage, so now it breaks my projects because of recommendations of the golang team.