Margarine will work fine, although if you're talking about the crust, we'd use half vegetable shortening and half margarine. (We'd use the shortening anyway, even if we were using butter. Shortening makes a more manageable crust.)
One problem with soy milks is that there's a pretty wide variety of flavors. We’re sure you can find one you like, but whether or not the first one you try will taste good in the pie is an open question, and you don't have a lot of time to experiment. Taste it before you add it to the filling to gauge how close it is to evaporated milk, and you may be able to perform some last minute compensations.
Soy milk tends to be on the sweet side, so you might leave out a bit of sugar, for instance. You might also increase the spices slightly to hide any off-taste from the soy milk. Finally, evaporated milk is thicker than soy milk, so a straight substitution may produce a filling that is runnier than you want. You might start out adding a little less soy milk than called for in the recipe, and increase it bit by bit if necessary until you get the right consistency.
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