Outer space is a perfect vacuum. Here on earth, at sea level, a gage may read Zero PSI (that's called 'gage pressure'). But in reality it is zero plus 14.7psi. because that's how much pressure is pushing on us in every direction. Get in a swimming pool and go 10 foot deep, or fly in an airplane... Feel your ears trying to POP? that's pressure, not much but enough, you'd need a gage with tiny marks to read it.
Most gages, for oil or water, we use PSI, Zero = press at sea level.
If working with steam or refrigeration, some NOT ALL, gages may have (psia) that's psi ambiant.
What is a perfect vacuum? Depends on who you ask. Or what chart you use. I say about 29" Hg. When the auto mechanic evacuates your Air Conditioner, he will lock in a vacuum of about that value. Since the air at sea level is 14.7 psia, if there are any leaks, the vacuum gage will move on his equipment.
The important thing is... that all the mechanics and engineers in your shop or field... are using the same figures. Kinda like being in an orchestra... Everybody's reading the same piece of music.