Question about vacuum vs pressure

We have a vacuum tank at work for mixing liquids. It can operate at -300 to -1000mBar. We are adding liquids from a tank in 800L batches.

The problem is connecting the hose directly on the tank will collapse it when it is empty. There is air intake at the top of the tank where we put 1,3Bar of pressure but this isnt enough, why is the vacuum so much stronger?

Currently depositing liquid in a separate tract and applying the hose there so when empty it just sucks air, but this is a messy procedure and requires a truck for moving and lifting tank up and down.

If possible we want to keep all tanks at floor level and applying the hose directly at the source without the tank collapsing.

At -300mBar and with 1,3Bar counterpressure will it work? The tanks are very expensive and the risk of imploding is not a risk we want to take.