As a guy who's been around the block a few times, using a vacuum cap to plug something as dangerous as gasoline lines is playing with fire, no pun intended.

If you are sure you are never going to use the line, and once again, I'm a BIG believer in vapor return systems, solder the thing shut.

WHATEVER you use, envolving neoprene, CLAMP it. If some hot weather/ vent problem/ fuel slosh event should dislodge your vacuum cap, you could have a problem. Don't endanger your expensive classic for a twenty cent cap.