Where can you find a vacuum diagram for a 1987 Nissan pickup?
That info is on pages 18 and 19 of the EF(Engine fuel) & EC(Emission control)section of the 1987 Nissan Truck/Pathfinder service manual.
You can do a search online using a search engine like Google, but the chances are pretty good that you'll have to borrow or purchase a mechanic's shop manual for that vehicle to get the vacuum hose diagram. Sometimes they're printed on stickers under the hood somewhere. You should check there first.