"the milk is there not for flavor but for the chemical properties, so I don't know if the soy stuff would substitute.
corn starch maybe? and if the filling gets too thick, add more of the preserve water"
the milk is, probably, not necessary
the idea is to thicken the sauce with liquid and starch, so a little vegetable stock would do perfectly well
corn flour is a very easy solution, but regular flout works well, as long as it's cooked a little first in butter/vegetable oil