A vacuum flask keeps warmth in for later, so dream books read it as preserving energy or affection through a cold stretch. It points to preparation and to carrying comfort with you.
Does the traditional reading fit your dream, or did it mean something else to you?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.