Soy milk does have a mild flavor, as does tofu. And it's important to determine if the individual is lactose intollerant, or allergic to milk protien. If he/she is allergic to milk protien, then you cannot use non-dairy coffee creamers as they contain the milk protien - sodium casseinate. Also, check the margerine label for the same ingredient. And you can alwasy replace the margerine with a good olive oil, or even a neutral nut/seed oil such as sunflower oil.
Roux works very well as a thickening agent, as do other starches such as arrowroot, tapioca flour, and cornstarch. For a pot-pie, I think I would use the flour-based roux, with chicken stock or broth to make a veloute. Add the diced chicken and veggies and you're ready to go. Make sure to thicken to the desired consistancy, and even a touch thicker than you want for the final product as the veggies will add some water to the completed dish. But thicken just a bit more. Also, add whatever herbs you might want, such as thyme, or sage, etc. to the veloute. Develop the flavor before you put it into the crust. That way, there's no guesswork involved.
Seeeeeeya; Goodweed of the North