Vinyl windows should open freely with a lot of force and maybe some oil if they haven't been opened in a while. You should also check to see if they have been painted and check the hardware (springs and rods) at the side of the window sashes for damage.
It may be because of the way the were installed. The installer may not have ensured all sides were level and may have also torqued to window. These may be difficult to fix. Some light oil on the frame may help.
I would recommend replacing hard to open vinyl windows. The reason that I recommend that is because if the windows themselves are old enough to have become hard to open, they probably also leak and are not air tight. This would cost you a fortune in wasted Heating and Cooling of your home.