Chinese subtitles/captions are available on some C-dramas when such source language subtitle files (known as “reference subtitles” to contributor teams) are made available to Viki (by providers) or when there are contributors available to caption each C-drama. To watch with Chinese subtitles/captions, you’ll need to switch your viewing language to your preferred Chinese format when available.
In the past, Viki had an awesome function - Learn Mode (ref 1, 2, 3, 4) - which facilitated language learning while watching dramas (only for C-dramas, K-dramas and J-dramas). Unfortunately, it was discontinued recently.