It has been pointed out that very long baseline interferometry (VLBI) is less accurate in determining the station positions in a vertical axis than in a horizontal plane. However, possible applications of VLBI for sea level monitoring, terrestrial reference frame for astrometry and earth rotation studies, etc., require accurate determination of the station position in the vertical axis as well as in the other axes. I propose three different approaches to improve the accuracies of the estimated vertical site positions, that is, to control the vertical elongation of the positional error ellipsoids by low elevation observations, to reduce the size of the ellipsoids by decreasing the residuals, and to select a "right" atmospheric mapping function to avoid systematic errors in vertical positions.