From: Rabbi Moshe Sofer
To: Rabbi Pesach
The Chasam Sofer rules that a dream vow is annullable before ten people, since its al da'as rabbim formulation is itself considered devarim beteilim. The vow's scope is limited to the upcoming Pesach, and the prohibition extends even to a keli sheni and cold water, as the concern is moisture-induced leavening.
Source: Shu"t Chasam Sofer YD §222