Try heating the butter and truffle oil and add the salt to that. If it does not dissolve then no harm done. You can add additional water until the salt dissolves. Look for a vegan butter with high water content. Soy milk and other vegetable based milks have fairly high water content. As pointed out in comments I am aware NaCl is does not dissolve in fat as fat is not polar. On the butter it is a package deal. The truffle oil needs to get mixed in so why not mix it in early and it will mix easier hot. Or start with moister potatoes then cook off liquid to get to the desired moisture.