Clearly the larger the diameter of the pier, the greater the stiffness and lower the vibration potential. For me however a thicker pier really gets in the way for imaging. I prefer to image all the way past meridian until the camera hits the pier if the target is far enough south so as usual you have two conflicting requirements. My first "decent" pier was an RTA traffic light pole which I mounted on four x 25mm threaded rods set into concrete floor slab. Vibration was never a problem I was aware of but the slab shifting with water content variations in the clay underneath remains a real problem especially with polar alignment.
Brian
www.bcoote-astro.com