Just noticed this little bugger (photo1) came off the Y section (photo2) of the vacuum lines. What is it? Where does it go? Does it cause boost to drop if it’s undone from the Y section? Is it vibrations that cause it to undo itself? How do I secure it so it doesn’t come off again; tie-clips?

 

Your car is 88+ so you don't have the venturi and the above diagram is for early cars. Take a look over where the A/C lines are and you probably have one undone under there as well.

 

Hard to tell, but that line looks just like the line on my car that comes off the large brake booster vac hose. That line with the blue collar runs over to the vac canister over by the battery, with another "T" connection running down into the heater/AC assembly under the dash. Or not.

Some people use zip ties to help secure their vac lines.

 

You guys are pretty warm.....

That check valve negatively pressurizes the vacuum cannister so that you will always have vauum available to keep your heater valve closed.
The heater valve behiind your head is "normally open" - so that you will always have (bad thing - can't turn it off) heat if your vacuum canister does't have a vaccuum supply from this pretty blue valve.

Keep the top of that valve conneted to the vacuum cannister by the battery.

 

ding, ding, ding, ding..........

 

what happens if the vac lines arent in that exact order?

 

Instant and absolute chaos - immediate universal collapse.....