Is there a way to lock the movemtent of vertices to a plane?

While doing symetrical modeling I would like all vertces  (of the object's certer/plane) to NOT move in the X axis.  This would keep the center/plane perfectly aligned and make welding a lot easier.  I always have a few vertices that get move off the center plane whic leads to problems.

thanks for any advice.

Attached Link: Aligning Vetexes

I'm not aware of a way to lock the vertices. However, you can easily align them to a drawing plane. Check out the link above to see how.