How effective is dettol liquid soap in vaginal disinfection?

Don't do it: Vaginal douching (flushing the vagina for hygiene or "disinfection") is never necessary and is a very unhealthy practice. Soap of any kind should never be used. If you think you need disinfection (e.g. discharge, odor), you need medical care to diagnose and treat it. These symptoms often are from STDs or bacterial vaginosis, and douching is especially risky if you have them. See a doctor soon.

Keep vag. life alive: A woman can just use the same mild shower soap that she washes everywhere else with, and just gently rub the places between the big labia and her thighs, and between the big labia and her small inner labia, and around the clitoris. That should gently remove excess oils (sebum) and stuff. Of course, "over-cleaning" can lead to rashes. The actual vagina is inside the body and does not need washing.