Fuel tank lines

[69GMCLonghorn]

I've got a '70 half ton with the "smog tank" for lack of a better word. It has the charcoal canister and the lines to the fuel tank (well, some of them are still there!). So lately the gas smell in the cab has been worse than usual. I don't see any visible leaks. What I did notice is on the passenger side, there are 2 lines that come out of the top corner of the tank and pass through a panel on the cab and go to the driver's side and back to the tank. One of these lines is open with no hose attached. I believe it's the line that used to go to the charcoal canister. Is it possible that enough gas is sloshing around that it's going through that line and dripping just enough to stink things up?

My plan is to re-install the canister (just because I want to!) and rerun the line. But in the interim, is there any harm in capping off that line? Will it create too much of a vacuum if I don't have a vented cap?