How to deal?

What's the right way to respond to a co-worker who askes "Did you ever have this problem with this person?"

In the last several weeks, I've gotten this question more times than I can count.  Sadly it's often about some of my favorite vendors, who after several years of working closely with I had to abruptly and uncermoniously pass on.  I don't know the right way to respond.

Given everything, a bitchy "YOU" probably isn't going to help - even though I know everyone in the office knows that's the best answer.  I even had to bite my toungue today when that was the response today from someone. 

I have politely said "I don't know.  I've never seen this side of them." and I've also taken the approach of coaching and trying to walk through the situation and how to better handle, which typically goes over like a lead balloon. 

Hello Rock - Hello Hard Spot!  Just another day at the office.