What is a vacuum bag?

A vacuum bag is a bag with low oxygen permeability and high anti-oxidation capability.

The polyethylene bags that we often see are made of a material that allows oxygen to permeate easily.
Therefore, even if you vacuum-pack with polyethylene bags, air will enter the bag within one night.

On the other hand, vacuum bags, also commonly called nylon polyethylene bags, are made of a composite film with nylon on the outside and polyethylene on the inside.
The nylon used for the outer surface has lower oxygen permeability than polyethylene, preventing oxidation inside the bag.
Polyethylene is used for the inner side because it is easy to seal (low melting point and easy to process).