This is not my idea, but one I saw on someone else's blog that I wanted to pass along in case you didn't see it (don't remember where I saw it!)
I rarely have unflavored soy milk on hand - we drink vanilla. If I have soy creamer on hand, I use that for my mashed potatoes. But someone said they use Veganaise. I tried this last night, and I can't tell you how good it was! I used a good spoonful for two large potatoes, along with some of the potato water. I also threw a couple of garlic cloves in when boiling the potatoes. Definitely mashed potatoes worthy of St. Patrick!