If you're stuck, it's time to get a smoke machine. I recently go the Autoline Pro and love it. It helped me find a vacuum leak I couldn't find.

My guess is that it is a piece of rubber tubing.
It allows you to connect the hard plastic hose to the hard plastic nipple on the vacuum canister.
If you look at # 19 in the diagram it looks the same.
Then look at the first picture in the first post. You can see #19 just below his thumb.
Bit of rubber tubing connecting a hard three way connector to a one way check valve.

Doesn’t it connect into the intake manifold?