sealing them with electrical tape will work, ofcorse.
i don't put mine vaseline+cottonwol tinder in straws, i find it hard to stuff it in the straws and harder to pack in a PSK, you can't squize them in tiny little space's left in a PSK, like the linyard hole and some portians of the air chamber of a whistle.... what you can if you don't pack them in something at all. And yes, the contents will get a little vasline on them, but does that matter? Not to me. Weather mine knife got a tiny amount of vaseline on them or not, its stil able to do it's job. just keep the stuff you dont want any vasline on them clear from the tinder.
i used to pack mine tinder in paperpackages ( waxed paper ). worked fine, untill i start using up the space in mine PSK..

just mine 2cents