No, don't use vertex manipulation. If you're trying to make the inside of the pipe, I recommend using displacements and subdividing them in order to achieve your goal (then surrounding them with a nodraw corridor.)
Essentially, create a regular square corridor (the height and width should be equal to prevent the pipe looking warped.) Divide the corridor into three segments, so that each segment is exactly one third the length you want. Select the inside faces using the Face Edit Tool and convert them to displacements. Click subdivide and it should round them into a pipe-like shape. Keep the middle segment, but delete the other two. Then resize the middle to fill the entire length. Surround with nodraw and serve.