The big plus with using vegetable broth to make mashed potatoes is that it adds delicious flavor, making it my top choice for a dairy-free mashed potato. The problem with most plant-based dairy is they often have a significantly different flavor that I find doesn't work well in recipes like mashed potatoes. If you opt for using plant-based milk, be sure to choose an unsweetened version and one with a mild flavor. Another option instead of vegetable is to use the cooking water from the potatoes and mix that into the mashed potatoes.