You should probably go and read the other threads on MBB, mylar and heat seal foil before posting mate, no offense but an informed opinion is always better than a subjective one. As you correctly stated though the best stealth is simple, thin and lightweight so if you are ordering smallish quantities of powders you want the packaging to draw the least amount of attention and have the layering have the lowest MVTR as possible, something you only get with a non permeable layer like a MBB. You could try and use 3 layers of vac seal but why make it thicker, more bulky for the sake of using an inferior material when you can sling the zip lock baggy into an MBB or heat seal foil bag and have an MVTR of 0.00002. If the envelope is singled out for extra attention e.g. scanner / x-ray then its gone anyway so your best chances are to avoid being singled out in the first place and that normally only ever happens when a dog reacts to it. Vac seal is outdated and doesn't work as effectively as other methods, that is fact not fud.