It sounds like you're proposing to use two gauge pressure transmitters, one at the top to measure the vapor pressure, the other at the bottom which sees the liquid level head pressure AND the vapor pressure and then subtract both readings to get level.
Yes, it can be done, but with an expected error that will be much higher than if you use a DP transmitter with two ports, a high and a low side.
A level measurement of say, 4 meters water column (wc) of liquid level, is a maximum head pressure of only ~0.4 bar. But you need two transmitters capable of static pressure 4 bar range (~40m wc head pressure) because of the static pressure of the closed vessel. The entire span of the level (0.4m wc) is probably only 1/10 of the range (40m wc) of the transmitters you're using.
The range of a DP fits the level measurement because its range is the subtracted value range, not the static pressure range. A DP does internally what you propose to do with two individual gauge pressure transmitters, but it does so without the combined error of two separate transmitters that are both ranged to be able to read the static pressure.
A DP compensates for the common static pressure by its internal design, so that its range more closely matches the level span, not the static pressure span.
