Locking Vertices to an Edge?

Hi, I’ve just started using Blender, and just started 3D modeling. I’ve been messing around with it and have been really enjoying myself. I’m mentioning this in case I use improper terms or have a problem conveying my question properly. At any rate…

I’m trying to take 4 vertices and scale them along the Y and Z axes, closer together. I know how to lock to just those to scales but when I do this, I get a dip in my model, since my model is “rounded”. Is there any way for me to move my 4 points closer, along only the Y and Z axes while keeping them locked to the X and Y axes of my original edge. This way, no dip is created in my model.

Thanks in advance!

And please, if I’m not conveying my point, I can try to post pictures to help. I’m super new.

And please, if I’m not conveying my point, I can try to post pictures to help. I’m super new.
Please do and also upload your blend file to http://www.pasteall.org/blend/ and tell us the download link