This is one of two lines going to the vacuum canister. I assume it is the vacuum supply line, but I can't find any fitting within reach on the vacuum manifold. Any ideas?

 

Yes, that's the check valve for the vacuum reservoir, the hard black plastic vacuum line has pulled out of its rubber end fitting
It will be there.............somewhere, looks like a "wire" so your brain/eye often over looks it as what you are looking for

It would have had to reach to the check valve so feel around, cold engine, for a loose "wire" end, it may not be a dedicated line back to manifold, might be on a "T" fitting

And worst case you can always make your own vacuum tap to existing source, but old "lost" line is a vacuum leak at this time

 

I hope this picture helps better....thank you