moving vertex along normals by code

if you ever worked with 3d painting digital sculpting stuff, like what you have in maya or even better zbrush (don't mention it's 2.5 because i know) or mudbox, you know that they give you sort of brush that can pull or push vertices along their normals. I am developing a mesh procedurally without a problem, i can use a brush to pull up or down the vertices on the world's up vector, what i can't figure out is how to move those vertices along their normal axis, can anyone help me and guide me toward an article or a code snippet on how to do it? thanks