We've had blimps filled with hydrogen or helium for awhile. And balloons filled with hot air even longer. There's noise about vacuum ships, which use vacuum instead of lighter-than-air gas, but they have to have a skeleton that can resist the pressure. Airships should be done by a very strong rigid skeleton that gets very fine near the surface but is big beams in the interior.
You can combine these. For example if it's at half pressure, and 600 degrees, and half helium, that's 1/2*1/2*1/2=1/8th the weight of normal air. There are diminishing returns of combining them.
Air+N2 won't burn, N2+CH4 won't burn, CH4+H2 won't burn, so a layer of N2 surrounding a layer of CH4 surrounding an interior of H2 is tempting. And it would insulate the H2. But if you got a hole and the H2 started burning in air, convection from the heat would accelerate the mixing, so such layers are only metastable.
Hot gas, if it were perfectly insulated, doesn't require extra energy to stay hot. Surface area (rate of radiation) grows with the square of radius and volume grows with the cube, so good insulation gets better the bigger you make the ship. Wind resistance is similar. This argues for cloud-sized ships with hot air interiors.
Air pressure drops the higher you go, and lifting power per volume is proportional to outside air pressure, so it's easier to make these if they fly low.
If it is a semi-vacuum ship and the skeleton can resist a certain amount of pressure, you can go down by pumping in more gas and up by pumping out gas, until you reach a level where the outside air pressure equals what the skeleton can resist and the interior is vacuum. It would have equal lifting power at any altitude as long as the absolute pressure difference is the same.