Both.
VNM doesn't offer any "formulation of rational behavior". VNM says that a function with a particular set of properties must exist and relies on assumptions that do not necessarily hold in real life.
I also don't think that a utility function that can condense the risk preferences into a single scalar is likely to be accurate enough for practical purposes.
Can you by chance pin down your disagreement to a particular axiom? You're modus tollensing where I expected you would modus ponens.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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