Cool and dry are good things for ammo. Consistancy of temp is very important. Ammo cans sealed with a bit of dessicant (silica gel packs) stored in a closet inside would be an excellent choice. I have shot some of my reloads from my college days (pre-84) and have had no problems at all. Ammo was stored in .50 caliber can with dessicant. Much is not most WW-II ammo is still good today as long as it was stored properly.
Vacuum packing is not needed. After a time (6 months to a year) you will notice the nice tight vacuum packed ammo is no longer vacuum packed. Air inside the case will actually leak out and into the packaging. Simply sealing the packaging to keep out moisture will be a better bet.