I don't see why T'Ryssa has to be played by an Asian actress.
Because her name is T'Ryssa Chen
. Her human ancestry is primarily Chinese. And because p. 68 of Greater Than the Sum
explicitly refers to her "tomboyish Asian features."
I originally thought her Chinese half was apparent (reading GTSS), but now it is clear that everyone confuses her with a full Vulcan (LtP).
How is that inconsistent? We've seen "white" Vulcans and "black" Vulcans, so why couldn't there be Asian-looking Vulcans? We've seen at least one full Romulan, Caithlin Dar, played by a Chinese-American actress, and there have been Asian extras cast as Vulcan background characters.