Erm, I could tell you that there is a 'Parking Riponne' right next to the square, but the chances of you getting anywhere near it that day are zilch. There are other car parks in Lausanne but if the event attracts a crowd, it's probably best to take the train anyway. In my opinion

I live about 20 mins walk from Riponne and will be using my feet to get there. Hope to see you too!