I read some of chinese article, one of the tactics PLAN might want to employ is the "killing by mistake".
Because the sub is hidden unlike surface warship flying a country flag. You know where it's coming and going. But sub is different.
If PLAN is certain a hidden sub is from Vietnam and lurking, the Navy would torpedo, or depth charge it and killed the sub, and then later apologize for killing it by mistake because of unkown origin of sub. PLAn would say it's a mistake as it think the sub is trying to do ambush.
Kind of like I kill you in the dark first because I don't know who you are and I think you are trying to harm me in the dark.
So, be very careful about hiding/lurking in the dark thing... Give people excuse to kill you.