Unfortunately, I think that ascribes too much power to VNM utility functions (that term itself is a LessWrongism; elsewhere, they would be called cardinal utility functions or just utility functions).

I actually don't recall seeing the usage "VNM utility functions" on less wrong at all, prior to this thread. It may have occurred previously but certainly not with sufficient frequency as to be a 'lesswrongism'. As you say, the "VNM" is unnecessary in that context since is all the VNM part does is say "it must have a utility function because it adheres to these axioms".

It is sometimes necessary to explicitly refer to things other than 'utility functions' with a 'VNM' qualifier. This is largely to pre-empt pedants who, when reading unqualified usage 'consequentialist', are not willing to assume that it refers to the only kind of consequentialist that is ever significantly discussed here (those that have utility functions).

VNM utility also falls flat on its face unless we already know what we prefer.

Not quite, but the point stands. The actual requirement is that there is any way to collect any evidence at all about our preferences (or, to be even more general, any way to cause outcomes to be correlated to our preference).