A vacuum around a pendulum is not all good.
In all practical tank clocks there is a technician who has to regularly check the vacuum
and pump it down to the mark. The clocks seldom attempted a hard vacuum. A perfect
vacuum has problems, touching metals weld themselves together. Lubrication requires oils,
and oils have a vapour pressure. (Some are lower than others, eg Apezion grease).
Incident radiation (light) will heat components and without air, dissipating heat is more difficult.
A synchronome may be accepted into you living room, but a rotary pump and high vacuum
system might not.
The vacuum will give you a much higher Q, and a high Q enables much higher accuracy.