vacuum hose

Hi all, I was wondering if someone could help me work out where this line goes - it looks like a vacuum line and there does seem to be some suction when the engine is on, but for the life of me I can't see where it should go. This is a C16SE. The vacuum line starts at where the yellow circle...

What internal diameter do i need for my LET as want to replace the mis-mash of hose there at the moment