How does insulating glass use vacuum or gas to reduce heat transfer?

[zuz]

What is the temperature of a vacuum? Could this be used as an insulator in windows/homes?

 

[BvU]

Thermodynamically, temperature for a vacuum does not exist (there is nothing that can have a temperature).
In windows/homes it isn't practical to maintain a vacuum just for its insulating properties: withstanding the forces from the atmospherical presssure would require a massive construction.
Real vacuum flasks aren't really vacuum, they just lower the pressure between the bottles

 

[Gigaz]

Depending on context, a space with only black body radiation is often interpreted as a vacuum with a certain temperature T.

 

[BvU]

Could this be used as an insulator in windows/homes?

 

[davenn]

Could this be used as an insulator in windows/homes?

It already is. Some window double glazing (2 sheets of glass vacuum or air between) methods already do this
Air is a poor conductor of heat

from wiki

Insulating glass (IG), more commonly known as double glazing (or double-pane, and increasingly triple glazing[1]/pane), consists of two or three glass window panes separated by a vacuum or gas filled space to reduce heat transfer across a part of the building envelope.

Insulating glass units (IGUs) are manufactured with glass in range of thickness from 3 to 10 mm (1/8" to 3/8") or more in special applications. Laminated or tempered glass may also be used as part of the construction. Most units are produced with the same thickness of glass used on both panes[citation needed] but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit.

Dave