Can we say that the volume of gas in a vessel is equal to the sum of the volumes of its molecules?

Molecules are unlikely to resemble perfect balls, cubes, or other regular geometric bodies with clear boundaries, so that one can strictly speak about their volume. One can start from the distances at which the molecules begin to interact with each other, and from these distances one can estimate the conditional volume and packing density.
It is known that gases are relatively easily compressed. This indicates that there is a certain distance between the molecules, due to the reduction of which compression is possible. The distances, of course, are average, because the molecules are in continuous motion: they collide, scatter.
At a certain pressure and temperature, the gas can be converted into a liquid state, and further compression requires enormous pressures. Most likely, the packing density of molecules in a liquid is close to the maximum possible and is approximately equal to the sum of the volumes of molecules. On average, during the transition of a substance from a gaseous state to a liquid, the density of the substance increases by about 1000 times. That is, about a thousand times the volume of molecules in gases under normal conditions is less than the volume occupied by the gas.
Answer: You can’t.