We currently have bowls of apple-cider vinegar set out, and the flies are definitely attracted to them but very few are actually ending up IN the vinegar. Mostly they are just sitting on the bowl.
What are other pesticide-free ways of killing fruit flies?
Open bowls don't work very well, what you really need is a bottle with a narrow neck, like a wine bottle. You put the bait (your apple cider vinegar or whatever) in the bottle, just a little at the bottom, and the flies go in, but can't manage to get out again. You can use any bottle or jar, but one with an open neck will need you to make a funnel to narrow the opening. Read this article: How to Get Rid of Fruit Flies.
In my experience, leaving the dregs of some fruit based alcohol like cider in the bottle works really well - I did that once unintentionally with a Babycham (pear cider) bottle and found it infested with fruit flies in the liquid at the bottom after three days when I discovered it again.
It's the surface tension in your current setup that keeps the fruit flies safe. Even if they decide to party on the vinegar, they can simply fly away again and take a nice break elsewhere.
But your trap becomes a real trap if you add a tiny drop of dishwashing soap: they can no longer walk on the vinegar, but sink and drown.
Using this method, you can use a range of substances as bait, from leftover wine to fruit juice or vinegar. Ideally, you have a mixture of fruity and acidic (e.g. equal parts apple juice and vinegar), because that's what they really love. Just make sure you use not more than a small drop of soap (or touch the bait liquid with a soapy finger), because too much soap scent will deter the flies.
I know the "funnel method" is often recommended, but apparently we had particularly stupid fruit flies in the past: after desperately buzzing around for a while without finding the entrance, they usually gave up and opted to raid the fruit bowl instead.
The trap as described above works even right next to the fruit to lure them away.
We make a simple paper funnel by just rolling a sheet of paper into a cone and stapling or taping it. Then we make sure there's a small opening at the bottom. We put this into a tall glass that has a little juice or wine in it - even a little bit of chopped up fruit sometimes. This works marvelously.
Also, check your drains. Fruit flies breed in a biofilm. When we had a very bad year, taking a bottle brush to the sink drain in the main floor powder room made an immediate reduction in the fruit fly population.
