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.
Make sure they were installed correctly and not to tight so they won't open. Run bees wax along the canal that houses the window and move it up and down.
Well if they are not opening properly, then just apply some vinegar onto the surface and rubbing alcohol as well to open them easier. They should slide open now and easily.
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.
