Why did my vegan croissants come out flat?
If they're your first batch, you did great! Can you show us a cut cross-section? A croissant will have "intra-layer" (inside the dough layers) rise and "inter-layer" (between the layers) rise: The intra-layer rise comes from the yeast and moisture action in the dough itself, and the inter-layer comes from the steam generated as the moisture in the butter/margarine integrates with the dough. I would guess that you didn't get a lot of inter-layer rise, but that's just a guess.

I use the Tartine recipe for croissants (with regular butter), and from memory you work a small amount of flour - say 10% - into your butter before forming the butter sheet. I have seen plenty of recipes that don't do this, but I'm wondering if it would help to stabilise the margarine a bit during the bake.

One other tip, make sure everything stays cold during the laminating. I put my sheet into the freezer for 15 minutes between turns, and I know some methods call for a couple of hours in the fridge (which presumably helps with hydration as well as keeping things hard). If the layers are cold, they stay distinct, which means they separate cleanly during the bake.

Last thing, I think you could bake hotter and perhaps double-glaze (glaze half way through final proof, and then again right before you bake), to bring out the colour on the outside shell.

Good luck!