I have a vegan couple I'm working with right now. It's been real fun
Honestly as far as cakes go I found that the best recipes are my own. I did visit several vegan websites and tried some of their recipes; NEVER again, just trust me on that one.
I use my regular recipes and I just sub Ener-G egg replacer for the eggs which works wonderfully although it is a bit weird, soy or rice milk for the milk and vegan soy butter for the real butter. If your using chocolate (I converted the chocolate mud cake recipe to vegan for this couple so they could try it) Sams bittersweet baking chips are made of soy, but you can also purchase very good quality vegan chocolate though it is more expensive.
As far as how good the cakes were once the recipes were converted I saw NO difference in the taste or texture. My husband who had previously mentioned how nasty that was couldnt even tell the difference when I had him taste both the vegan and non-vegan recipes.
And as the above poster mentioned you cannot use regular marshmallows to make marshmallow fondant. There are vegan marshmallows but they are very very expensive and I can only find them online. My couple is going with my vegan friendly buttercream recipe and regular fondant for their decorations.
Really its not that hard and you dont have to go sleuthing around for crazy vegan recipes to make the cake. Use your own and just use vegan substitutes for the non-vegan ingredients. Couldnt be easier.