Is lemon/yoghurt/honey/coconut oil safe for vagina lightening? if not, what can i use?

No: The genitalia are normally darker than the skin on the rest of the body, unless one has a dark tan on other parts of the body. It would look unnatural and strange if one's genitalia were the exact same color as, or lighter than, the rest of the body. Again, it would look unnatural and strange if one's genitalia were the exact same color as, or lighter than, the rest of the body.