You think you can stab a knife through a sheet of kevlar? Good luck to you there matey.
You can stick a knife through the gaps between the plates of kevlar certainly, but not through the bullet proof slabs.
How do you vaporise the slug? Now you could just use 'handwavium', I've got no problem with that, its soft sci-fi and also you to get on with the story.
If you want to explain it you've got to say what happens to all the KE of the slug. It cannot vanish, and you've got to put some energy into vaporising the slug, thus chuck enough material at it and you'll overload the shields.
Everything we have ever seen, at every level, in every field from quantum physics through to cooking to parallel dimensions and black holes supports 'Energy can neither be created or destroyed.' It's fundamnetal, from comet orbits to falling out of a plane,
everything obeys this. That's why I can say those shields will never work as you say, they will be vulnerable to slugs. You cannot just wave away all that energy.