I do some vegan cooking at work and have to admit it is a little hit or miss. In some recipes almond or soy milk work fine. Coconut milk can also be good because of the added fat content, but I don't believe you will find an absolute substitute rule of thumb (I have looked). You shouldn't have to change the quantity, but you will have to experiment a bit.
If it is only lactose you are avoiding, you can find some high fat butters that have almost no lactose, however if you have to avoid dairy products altogether I find that Earth Balance brand spreads work well in place of butter. They have a variety and some are softer than butter, so you need to make adjustments in pie dough and such.
Coconut oil and a little agar make a good "cream" if you need a pastry cream-type filling. I will bind the agar with soy milk or fruit puree. Cool and blend in Robot Coupe while emulsifying in warm coconut oil. I usually like the option with fruit puree or will add lots of vanilla bean and a little lemon zest to hide the coconut flavor. The mouth feel is very nice and it freezes well.
The information out there on vegan or dairy-free cooking is limited. I have found a lot of truly awful recipes and have to test a bit to find ones eligible for a fine dining atmosphere. I have a few decent cookbooks, but they are aimed at home cooks. If anyone writes a professional-level cookbook on the subject I would expect the demand to be good.