man those "I swore I told em now I can't find the thread" items are the worst... or the verbal miscommunications like what it sounds like you may have had where there just is not trail to look up for facts...
killers....
harbingers of stress and grief...
I had one of these come up recently... I know I told them.. problem was it was verbal.. I thought I might have it in pixels somewhere too but I can't find that ish for the life of me..
it's basically information that means they now have to hire a whole person to do a job that they were perhaps assuming they wouldn't need someone for based on their understanding of what I had laid out.. I swear I told them this is the caveat... other than this caveat your gold but there is this one caveat.. you have to plan for this....
I told them.
I can't prove I told them.
Good news is I think they're just accepting it for the reality it is and moving on rather than complaining about it.

Hope this vendor cuts you a good break and that your boss doesn't flip out. good luck.