What is the extra hose on the diesel tank

[Bon Voyage]

Hi, on my 2001 bus I have the main fuel filling hose that goes to the tank that looks like it's about 2" and beside that I have a second hose that might be 3/4" It ties into the main 2" line near the end where I fill the diesel in and as far as I can tell the other end goes back to the fuel tank.

So my first question is does anybody know what this smaller line is or what it does? (my guess is a vent somehow)

My main question is can I tie in a return to tank line in this from the FASS pump I'm installing? (This is a aftermarket fuel lift pump that separates water, air etc - it requires a 1/2" fuel line going back to the tank that will have a constant flow of diesel. So I hope I can tie into this unknown hose for the return)

thanks