My thoughts are that vac packing ammo is a great idea for ammo that you will depend upon, especially bugout ammo, as opposed to dedicated range ammo.
The other justification for vac packing ammunition is its rapidly escalating cost, so consider it a step further in protecting at least some of your investment.
12GA shotgun ammo is especially susceptible to humidity, so I vacuum pack most of my Brenneke stash.
I use our Food Saver. One hint: Use only Food Saver or Rival bags, which are quite costly. The generic bags (from Walmart, for instance) are junk, forget the fact that they cost less, they crack at the folds and leak.
Besides cost, one other tradeoff - You can't vac pack the ammo and expect it to fit tightly into ammo cans. The necessary excess bag mat'l is bulky and shouldn't be crammed or kinked severley. But if your basement floods, or your pack gets soaked, etc, your powder stays dry.
Actually, this is the only time of year that I vacuum ammo. The interior humidity level in my basement w/ furnace running and below freezing outside is around 15-17%, so what I put into the bags is at its driest.