Putting aside shrapnel effects, I believe that high-explosives cause damage by producing a shockwave. How do shockwaves work in space? I've managed to convince myself that a high-explosive shockwave would work "about the same" by producing a shell of high-speed gas that would arrive at the surface with pretty much the same amount of energy that you'd get in air. Is that right?
What is a shock wave?
This about covers it.
Your thought experiment speaks of an explosive, and it can create a shock wave only if a medium exists to be shocked: sudden almost instantaneous creation of vacuum in air/medium and immediate collapse of that as air/medium rushes to fill it.
The answer to your question
is no, it is not right.
To reply to the title question:
one would have to know the number of shrapnel from the explosion, and also if there is dust from the explosive or gas for some reason. From conservation of energy the fragments in vacuum will have larger energy because they will not be pushing against a medium ( to create the shock wave) and if they hit would do more damage. Any dust and gas would also move faster but one would need a detailed model and objective for the explosion to know whether the damage would be the same as in non vacuum.