If your company is hoping to keep up relations with that vendor, I do agree that this is a trainwreck waiting to happen.
However, this is not your decision. It is your manager's. Maybe they misunderstood how deep the issue goes. Maybe they're not understanding that the vendor won't just take the feedback and improve on it.
Talk to your manager. Something along the lines of:
Regarding the issues we've had with this vendor, they run very deep. These aren't minor flaws that can easily be corrected, they show a deep rooted bad practice to creating/delivering their product. It may be more efficient to find a new vendor rather than wait for them to turn everything around, though I'm aware that decision is not up to me.
What is the expected outcome of this meeting? Are we trying to continue working with this vendor and try to address as many issues as we can without souring the relationship? Or are you expecting me to give brutally honest feedback, but not guaranteeing that they still want to work with us in the future? I currently don't see a way to both be open and honest, while also guaranteeing a good future relationship with this vendor.
You need your manager to decide here, because there may be plans that you are not aware of.
For example, maybe your manager is hoping for a brutally honest review so they can then use that as leverage for breaking a contract or not paying the full amount (as the product delivered by the vendor did not meet the requirements of the deal).
Or, alternatively, your manager is naively assuming that the vendor will always try to please their customer and thus drop everything to fix the things you list as wrong.
Or, as a third option, your manager is unsure whether to continue with this vendor and wants to see their response to your feedback; are they apologetic or dismissive, are they open to fixing things?
You can't know this. So ask your manager. The manager decides the priorities: honesty or keeping up a good relationship?
