The same as center_blit(), but only centers vertically.

def vcenter_blit(target, source, dest = (0, 0), area=None, special_flags=0):
    '''
    The same as center_blit(), but only centers vertically.
    '''
    loc = lambda d, s: (_vec(0, d.get_height() / 2) - 
            _vec(0, s.get_height() / 2))
    _blitter(loc, target, source, dest, area, special_flags)