Vettel reportedly said that the "noise of the engine is shit" following the introduction of the 1.6-litre V6 turbo-charged power units this year.
FIA president Jean Todt wrote a letter to the four-time world champion warning him about his future conduct, and Dennis also believes that the German acted disrespectfully.
"Being a world champion requires a dignified approach to everything, so putting aside the language, even the sentiment is inappropriate," Dennis is quoted as saying by PA.
"But the simple fact is if he was sat in a Mercedes he would be extremely happy, and I'm quite sure any four or five-letter words would be more of joy.
"He should just reflect on the fact he has had a period of dominance, and just because that dominance is being shaken by Mercedes, it doesn't give him the licence to be disrespectful of the obligations placed on him as a world champion."