Changing the alpha of specific vertexes:

How would I go about editing the actual vertexes of a plane? For instance, I'm trying to implement fog-of-war in an RTS game. What I've decided to do after a considerable time spent researching is to make a plane above my units but below my main camera. Then, if a unit walked under it, set the vertices to alpha 0 by using a raycast up from the object to detect where the unit is. However, I don't know how to change the alpha of specific. How would I do that?

Thanks in advance, Elliot Bonneville.

P.S. Thanks for anwering all my many questions, everyone!

You would need to make use of the Mesh class, in particular Mesh.colors for vertex colors. Also you'd need a shader that actually uses vertex color of course. Most of the built-in ones don't, but the particle shaders do, and there are some on the wiki as well.