Q.treating white powder on gardenia
I was told to treat the white powder by applying soapy water and oil. I have a Gardenia plant and it also got into my night jasmine. Is that a good house remedy? My plants are planted in vases.

Sounds like you have powdery mildew which is very contagious in the plant world. I would isolate the plants that are affected with powdery mildew to prevent further spread and treat the affected plants.
For more information on powdery mildew and for treatment options, please visit the following article:
https://www.gardeningknowhow.com/houseplants/hpgen/powdery-mildew-treatment.htm