We don't recommend putting anything in the water, because it changes the taste and many pigs will not drink as much. Also, Vitamin C degrades quickly in water and when exposed to sunlight, so after an hour or two in the water bottle it's pretty much useless.
Oxbow young guinea pig pellets do have SOME stabilized vitamin C added into them, so she's not getting nothing.
If she doesn't have another pig housed with her to show her how to eat veggies, you can try chopping some veggies up - like REALLY finely, minced - and mixing them into her food pellets. The hope with this method is that they get used to the smell of the vegetable and hopefully they accidentally get enough of it into their mouth to get used to the taste and learn that it's food. Do one veggie at a time over a number of days when trying this method. Also, try whole leafy greens like green or red leaf lettuce. Sometimes pigs that haven't ever had veggies before have any easier time accepting leafy greens.