1. bubbler
I presume a vacuum pump is pulling the vacuum.

If the vacuum can tolerate a small amount of air, then a bubbler with an ultra low flow constant flow regulator can supply a low flow of air (or bottled nitrogen if air is problem) through a dip tube/pipe inserted into the liquid.

A differential pressure transmitter high side connected to the dip tube with its low side connected to a 2nd pipe/tube that connects to the non-liquid vacuum space will report the level in units of water column: inches or feet of water, or mm or m of water.

Level done by differential pressure assumes a constant density. Any change in density from the design density results in a proportional error in the reported.

2. Ultrasonic does not work in a partial vacuum.

3. Non-contact radar or guided wave radar will work in a vacuum.

Thank you danw2. The vacuum is created by a centrifugal blower that draws gas from the landfill.

If you pursue bubbler technology, be aware that a tube perpendicular to a flow stream with the flow flowing across the tube opening is subject to a venturi effect and will read a lower pressure than the real vacuum pressure. A lower-than-expected pressure on the low side of a DP transmitter results in a reported level output with error on the high side. The error is likely to be constant when the pump runs, so checking the level between when the pump (blower) runs and when it doesn't (assuming the level doesn't change at that moment) will show you the magnitude of the error.