Probably best not to refer to it with the word "utilitarianism", since it isn't a form of that. Calling it "consequentialism" is arguably enough, since (making appropriate asumptions about what a rational agent must do) a rational consequentialist must use a VNM utility function. But I guess not everyone does in fact agree with those assumptions, so perhaps "utility-function based consequentialism". Or perhaps "VNM-consequentialism".