Personnally, I would avoid the area between Place de la Riponne, Place St Laurent and the Bel Air area, especially if you are alone and a woman.
Place St Laurent has always attracted strange people, but there are usually police around.
Usually nothing happens, people just convey an uneasy feeling. But I have known of rare cases of light agressions, so if you have the choice, just walk around the area. That's what I do.
We had a very bad experience in Lausanne with our kids. I would especially avoid the Creperie d'Ouchy there. We were asked to leave before them even serving us because our 14-month old daughter was hungry and was crying. Apparently that is not appropriate in a Creperie in Lausanne at late lunch time!!
Just two things : Montreux Jazz Festival is a very safe event, this meens don't take a pocket knife with you (trouble with security guys) and also no camera (that's worse than the first one...).
When you arrive there, you have to change your swiss francs to Jazz coins (1Sfr = 1jazz). Don't go to first change office, everybody will take this one, go a little bit into the festival and you will find plenty.
Mountain air is lighter than ground level air.
And it looks like your legs also gets heavier.
Don't take on more than you can chew, you can stay there a while if you can't climb back down.
Here is Louise that is wondering if she should go on, stay there forever, or go back!
If you don't work out before coming here and you're not fit, your muscles will be killing you after wandering Lausanne's steep streets.