Sometimes when I vacuum, the room ends up with that stale, musty odor. So I added about a 1/4 cup of baking soda inside my vacuum bag. I just sprinkled it inside the round opening of my bag and reattached the bag to my vacuum cleaner. And off I went vacuuming.
The result was a pleasant, odor-less smell after I finished vacuuming. Baking soda has long been known for its odor absorbing abilities and it works well in a vacuum cleaner bag too. Oh and one more tip, buy your baking soda in a 4 pound box as shown. It is much more reasonably priced that way than buying the smaller, more expensive little boxes.