It is illicit to not wear a chasuble if there is one to be worn. With that said, I’ve never been to a mass in which the priest wasn’t wearing one, but I’ve seen plenty of pictures.
I’ve been to two masses with a deacon without a dalmatic, but only in the case in which there was more than one deacon assisting the priest and there was only one dalmatic. In such cases, the deacon dealing with the sacred vessels at the altar wore the dalmatic, and the one reading the Gospel did not. It so happens that sometimes at my parish, for whatever reason, two deacons are present at masses, kind of like when there used to be subdeacons for high masses.