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Most of the time citric acid is used as an enhancer to fruits to make them "pop" but it's typically a 10% dilution used at .25-1%. It won't come across as sour but when combined with malic acid it does create a mediocre sour sensation.
I wouldn't use it to flavor straight VG as it's unlikely to dissolve easily and probably wouldn't be much more enjoyable than just vaping unflavored VG.
Well I do enjoy vaping straight VG, would it add ANY noticeable flavor?
It's much more likely to add tons of throat hit at the level you would need to get any kind of flavor out of it. If you're looking for a cheap and local solution to adding flavor you can try to find some LorAnns flavorings at places like like Hobby Lobby or Walmart. LorAnns Banana Cream @ 5-10% would make a much better single flavor than citric acid.
It would be safe, if the citric acid was pure. Not sure if you'd get it pure at a store. It also wouldn't do much. Citric acid can be useful in mixing, but usually at concentrations well below 1%. I'd have to check again to find the exact proper percentage... it does tend to vary depending on what it's mixed with.
You can always vape pure VG. It'll be ever so slightly sweet.
And if you have access to a Chem lab at school, you can do much more interesting things (without using any chemicals you didn't bring with you)... for example, soak a couple cinnamon sticks in VG, pour the VG into a bottle after a week, take that to where you can use a centrifuge. (You want to ensure you remove any solids, that's the most effective approach. You can also use filtration techniques, but I like the other way.)
This is it: https://imgur.com/a/Vyhp9