Is it safe to vacuum pack lighters, butane canisters or ammunition? Thanks in advance for your replies.

Welcome.

I have never tried it, but I don't see why not,

I doubt very seriously that the vacumm is strong enough to crush the lighter, pretty sure it couldn't hurt a butane canisters (although I wonder why one would want to seal one of those), and I am positive that it couldn't hurt ammo...

Ammo in large quantities is usually vacuum sealed, so, that should be a problem.

You might consider putting something that gets rid of moisture like a desiccant. Otherwise when you seal the contents you seal it in with moisture and you will get corrosion or rust.