There's no need for any vertex manipulation, all you need is an arch which is basically the exact same thing as a cylinder except that it's... hollow!
Here's how to make one:
1. Click the Block Tool button:
2. On the right, make sure Categories is set to Primitives and Object set to arch:
3. Now create an object like this:
Grid size is 16 (change it with the [ and ] keys), zoom is 0.50 and the sube is 128w 128l 128h (width, length, height). Hit Enter to create the object.
4. A dialog box pops up:
Here you can specify the number of wall sections and width. Take over the settings you see on the image. When you're done, hit Ok.
And that's it! A hollow cylinder:
From here you can change the height, but avoid changing the width and length because that would mess up the object(object vertices go off the grid) and it'll look bad ingame.