When to call DPW or MPD?

One of the ice cream vendor trucks has been parking on 4th and PA Ave next to gallery of art every day blocking the bike lane and some of the lane. I reported a parking violation via 311 and they replied and said it’s not their responsibility and to call MPD. I don’t understand that. They’ve handled parking in the past but that was in more residential areas so I’m confused about the rules. I find this so frustrating.