You are right, ViEmu has never supported the 'c' flag yet for :s.
The correct use is ":[range]s[ubstitute]/regex/replacement/[g]"
We plan to provide it in a future release.

Thanks

This command
    :%s/This/That/c

replaced all occurrences w/o prompting ( as though 'g' had been used).
Is this not working like Vim does, or am I misunderstanding something?

(I also tried
    :%s/This/That/gc
with the same result.)