I tend to substitute oil for butter in my recipes, but because butter has quite a bit of water in it I remove about 2TB per cup, so instead of 3 cups of oil I would use 3 cups minus about 6 Tablespoons. My Joy of Cooking recipe offers a range to the amount of butter one can add to muffins and this puts it somewhere toward the lower end of that.
I tend to use either water or rice drink instead of milk, but pretty much any liquid will work.

Help is on the way! I have a dairy-free pumpkin bread recipe that only uses vegetable oil. It's great! Try the muffins with oil. Lots of dairy-free butter substitutes at Whole Foods, even the regular supermarket has them sometimes.

I just posted a pumpkin muffin recipe that is special in our house, and it uses oil instead of butter. It does have milk in it but you could sub in orange juice and decrease the sugar slightly. I did that when I didn't have milk in the house and they turned out great.
http://www.food52.com/recipes/6974_birthday_pumpkin_muffins

Earth Balance is the best vegan butter substitute, by far. If you use another margarine, make sure it doesn't have hidden dairy ingredients like casein and whey.

A soybean based margarine will work fine! Spectrum and Earth Balance make them.