I am using Vs (stall, clean) since the configuration is the same as
at Va.

Vs1 is normally taken to be the clean stall speed. The meaning of Vs
seems a bit more ambiguous. But we're talking the same thing here.

How does CAS account for density altitude?

With a given CAS, the density altitude is irrelevant. That's the
beauty of it. ;-)

For a given DA, you could use TAS and get the same results, as long as
you're consistent in using TAS for each figure. However, Vs1 is only
published as CAS. And there's no point in doing that anyway.