Suffering from vaginal yeast infection. does washing the genital area with dettol antiseptic help reduce the itching?

No: No it won't, but treatment of the underlying intra-vaginal yeast infection will ultimately resolve the issue.

Vaginal yeast: Dont' do it -- could make things worse. If you're on anti-yeast treatment, you can use hydrocortisone cream to help with the itching. If you haven't seen a doctor to confirm the diagnosis, for sure don't use antiseptics -- and see a doctor or clinic soon. Good luck.