Liquid can be pumped into the bottom of vat, but simply reversing the pump direction doesn't allow liquid to be pumped out. It appears to move visually, being shown in glass sided pipes and creating particles at the end of open-ended pipes, but not actually moving fluid to containers or spilling any with open-ended pipes

{east: {OpenEnd: {Pulling: 0b, FluidName: "minecraft:empty", Amount: 0, Location: {Pos: {X: -31, Y: 66, Z: -66}, Face: "EAST"}}, Pressure: [0.0f, 256.0f], Flow: {FluidName: "destroy:mixture", In: 0b, Amount: 1, Tag: {Mixture: {Temperature: 313.8094f, Contents: [{Molecule: "destroy:mercury", Concentration: 32.86734f}, {Molecule: "destroy:hydroxide", Concentration: 0.68838894f}, {Molecule: "destroy:hypochlorite", Concentration: 9.238836E-6f}, {Molecule: "destroy:sodium_ion", Concentration: 1.1577386f}, {Molecule: "destroy:chloride", Concentration: 0.469286f}, {Molecule: "destroy:water", Concentration: 25.41025f}], AtEquilibrium: 0b}}}}, west: {Pressure: [256.0f, 0.0f], Flow: {FluidName: "destroy:mixture", In: 1b, Amount: 1, Tag: {Mixture: {Temperature: 313.8094f, Contents: [{Molecule: "destroy:mercury", Concentration: 32.86734f}, {Molecule: "destroy:hydroxide", Concentration: 0.68838894f}, {Molecule: "destroy:hypochlorite", Concentration: 9.23897E-6f}, {Molecule: "destroy:sodium_ion", Concentration: 1.1577386f}, {Molecule: "destroy:chloride", Concentration: 0.469286f}, {Molecule: "destroy:water", Concentration: 25.41025f}], AtEquilibrium: 0b}}}}, x: -31, y: 66, z: -66, id: "create:glass_fluid_pipe"}

I'm unable to replicate this. Are you able to pump fluid out the bottom if the pump was placed so it pumps out to begin with?

Piping out from the lowest of the side works, but can't get all the liquid out because it falls below the pipe's mouth. Maybe change the inner mouth of vat pipes to take an entire block face for convenience?

Tested it, it seems that it works correctly if the pump is connected straight to the vat, but fails to move fluid if it's connected via a pipe. This seems to be true even when pumping from the side, so it seems to not be a side/bottom problem.

@FlamingKetchup would you be able to send me your world so I can try and replicate with the exact same configuration?