The PCV valve on a 1988 GMC Vandura 3500 with a 5.7L V8 is typically located on the
valve cover. It's usually near the back of the engine, on one of the valve covers, but the exact location can vary slightly depending on the specific engine configuration. You'll likely see a small, usually plastic, valve with a hose connected to it. One hose will go to the valve cover and another to the intake manifold.
To be certain, you should consult a repair manual specific to your 1988 GMC Vandura 3500 with the 5.7L V8. A Haynes or Chilton manual would be a good resource, and you can often find diagrams online as well if you search for "1988 GMC Vandura 5.7L V8 engine diagram". Look for the diagram of the top of the engine to pinpoint the PCV valve's location.