A medium baking potato weighs about 180 grams and contains less than 1 gram of fat. All bread, especially wholemeal, is good for you. A lot of the fat in sandwiches can come from butter, margarine, mayonnaise, or oil based dressing.
Potatoes are an excellent source of starch, and if eaten with the skins are a good source of fibre.
Keep fatty baked potato fillings like cheese or sour cream as a treat and when you do have them, try not to have butter as well. Don't forget there are plenty of sandwich fillings which are healthier choices. Good choices for vegans include fillings based on beans or chick peas, such as humus, curried chick peas or bean salads. Peanut butter and tahini paste are good sources of protein for vegans, but are relatively high in fat, so go easy on these and ask for plenty of salad for a balanced filling.