If you examine a clear incandescent bulb, you will see that the support for the filament is hollow and has a tube inside extending down. This little "straw" is

If you examine a clear incandescent bulb, you will see that the support for the filament is hollow and has a tube inside extending down. This little "straw" is the point of connection for evacuation where the air is removed and then back-filled with argon. The argon coaxes the tungsten back onto the filament preventing rapid sputtering which would happen in a pure vacuum. After the argon the little tube is heated until it hits melting point and crimped off for a near perfect hermetic seal. The argon pressure is still well below atmosphere so the bulb is still considered under "vacuum" compared to atmosphere.