My own thought is imagine running a siphon in a large chamber and steadily evacuating it (to remove the atmosphere). At some point the water will vapour lock in the siphon, will it not?
It depends upon the liquid. For a Mercury siphon operating under these conditions - the mercury will vaporise from the exposed surface, but the mercury within the siphon tube will have sufficient pressure from the liquid itself to avoid boiling.
Apart from that, the siphon works just like a
slinky, or if you were to link together a chain of paper clips and place the chain in a paper cup. Take up one end of the chain and lay it over the edge of the cup, pull the chain so that the free end hangs down below the cup - once you have enough weight hanging down, the force of gravity on the hanging chain will pull the rest of the paper clips out of the cup.