Honestly?
"Cute" cuts out most of the Vocaloid cast if you actually use the true defination of it.
Thus your left with basically the younger Vocaloids in general... Ryutu if you consider him cute, Una, Oliver, Yuki. At a pinch Rin and Len.
Some people push "cute" to mean "attractive" and sexualise things, but I disagree with lolicon so I'm honestly going to choose between those that actually fit the true definition of cute.
Of the "cute" Vocaloids, the cutest to me is Una.