Literally 100% of that heat travels from the 1000000C stuff to the environment throught that vacuum. Vacuum doesn't just remove energy.
If you use a steam engines it doesn't matter if your source of heat is 900C or 1000000C, all heat will be captured, and 40-60% will be turned into electricity.
What you said there is all true, but largely because you didn't mention efficiency. If your heat source is a lot hotter than the steam you make, you do lose a lot of efficiency. If you had a million degree heat source, you could have many steps extracting huge amounts of power before your "waste" heat gets down to 1000C and is used to boil water.
The part about bad conduction being a problem is nonsense. The "lucky to get 1% efficiency" is not nonsense.
Carnot efficiency is 1 - Tc/Th, where Th is the hot side temperature and Tc is the cold side temperature. Tc is set by the surrounding environment, probably in the vicinity of 300K. If you have a hot side temperature around 1,000,000K then the theoretical maximum efficiency is very good. If that heat has to be stepped down by separating it from materials that would melt and you can only sustain a hot side temperature of 1200K, then your theoretical maximum efficiency drops to 75%. Obviously the real life efficiency will be a bit less than that, but the principle shows that the "lucky to get 1% efficiency" bit is nonsense - you're not actually losing that much after all.
This is all about getting energy out of a very hot heat source. Theoretical efficiency is ~1, and a ~40% practical efficiency also doesn't seem to be hard: let something heat up to 1000C, and don't let much of the energy escape to the environment.
Also deuterium-tritium reactors get energy out of the plasma via capturing high energy neutrons, very similarly to nuclear power plants.
