I've looked at the Russell website. They recommend the Proflex -4 lines for vacuum. Is there any reason I couldn't use some of my extra -6 line to provide vacuum to the brake booster? Does it collapse?

It probably will be ok for a while,but why take a chance with something designed for pressure and use it in a manner it was not necessarily designed for.If it fails and the pedal goes full hard because you have little or no vacuum, it might become a problem.Just my.02

I've used it on mine for twenty years .
Just check it periodically for age related deterioration.