I need vacuum line diagram for 78 GMC Jimmy 400

There's a sticker under the hood that shows the correct diagram. They are also shown in the manufacturer's service manual but there will be a lot of variations. You have to find the right one based on engine size, type of transmission, federal or California emissions, and other options.

Start with what? Are you re-plumbing the entire engine or just looking for where a loose hose goes? I can help with some of the specifics but I don't have any service manuals that go back that far. I don't even know for sure if the online service manuals like AllData and Mitchell have vacuum diagrams. Is the sticker under the hood missing or do you just not know how to read it?