Fruit flies breed in decaying matter. That means drains. Houseplants are a huge unsuspected source. If you use vinegar on plants, you get dead plants. Instead, use soapy water. Pour over the base of the plants. Then follow up with a rinse of clear room-temp water.
Repeat as often as your plant can stand. ie, no plant likes over-watering.
Drains? Could also try boiling water - kills fruit flies and also cleans out the greasy aspect of drains. No harm - and may well do some good. Again, repeat as often as you feel like doing it.
Dot