could it work without a big hit on performance? is there any shaders for it already?
no one?
its a common thing to do that in fixed function pipeline game dev as such definitely doable 
for doing it you would likely use the vertex color shader from the wiki / iphone standard assets
Well I need a shader that uses a light maps too.
in that case you can take the sources of lightmap vertex shader setup and combine them.
unity iphone 1.x does not support programmable pipeline, as such combining them will be straight forward
Well I’m working on doing art for a game that’s target specs are for the iphone, but its currently being made in windows and will be released for web, too. I don’t know much on the programming side, any good resources for learning how to work with/create shaders in unity?
you don’t need any programming for this. its done through the shaderlab commands which you can find in manual and reference (#cgprogram is not available in unity iphone 1.x and on unity iphone 3, itsa 3GS+ / ipad only thing to use real shaders and not needed in this case where you want to use vertex alpha)
You should be able to do all you need if you can get through tutorials 1-5.
wow!! incredible useful those tutorials Jessy 