Ah, it appears that was largely disabled if the Vive controllers were enabled and a game pad is active.
However, there doesn’t seem to be any way to map buttons when shift isn’t enabled. For example, I wanted to change the left trackpad to mouse wheel, but “change button mappings” gives only an option for center-click, and an option for what clicking the trackpad does if it’s shifted. No options for changing what it does if it’s not shifted, and no way to have a touch register as a mouse wheel selection.