I've done a report analyzing how a vendor had done a piss-poor job in our project. The report is basically an explanation why the vendor's technical choices are terrible, and supported by various objective analysis, stats of our performance data and fair comparison with our competitors.
I shared this report with my managers, who are convinced by my arguments (they are not technical).
The problem is that I am now asked to present my findings to that vendor in a web meeting, and I honestly believe their quality of work is terrible beyond imagination. Those two are conflicting. If I present in my honest way I would absolutely destroy the relationship between us, which has not been bad, and which we would need at least for a few more months. I would also get a bad reputation within my team as the person who condemns our vendor and create friction for future work.
Needlessly to say I wish we could seek out another vendor, from day one. Quite a few within the company share my opinion that the vendor's quality of work is not up to standard, but for one reason or another we end up continuing with them.
Usually I let my managers deal with the nice talking stuff. Apparently it was decided that this information needs to be shared with the existing vendor and I will be the one to do it. So I need to talk nicely, somehow.