We are regular snowbirds in Puerto vallarta, often for a 4 or 5 months stay in wintertime. I can assure you that during those years we never had any problems, not on the streets, beaches, shops, while driving car or bus etc.
With us thousands of canadians and americans are doing the same and they also having a great time. I know many places in the world -incl. Europe and the USA -that are more dangerous for tourists. Remember that almost all killings in Mexico are drugsrelated.
I still walk the streets in Vallarta at night. Friends from Canada do the same and as they state they wouldnot do this in their hometown....
This winter we also stayed in Mazatlan during a few days, and ...also there no problems. We had a great outdoor dinner at night togoether with a group of tourists but also many locals were enjoying it.