You shouldn't put vitamins in their water. It's a completely ineffective method as vitamin c doesn't last long in water, plus has a tendency to make the water taste funny which may stop them drinking as much water as they may otherwise. It's also an inaccurate method of administration to add anything to their water, as if they do need something adding in as you don't know how much they will drink and you don't want them having to too much or too little of it.
If you have a good quality pellet with vitamin c added, and give a good range of veggies, they should be getting all the vitamin c they need. I only ever supplement in times of illness when they need an extra boost. Keep offering all the veggies that are hight in vitamin c (have a look at this list for more ideas for veggies high in vitamin c
http://www.guinealynx.info/nutrition.html#order-c), and if you feel she needs a boost because she's not eating her veggies, then Oxbow make some chewable vitamin c tablets, that apparently many will eat straight out of the packet.
Has Peppermint got a piggy friend? Often solo pigs don't popcorn around as much as they are social animals who prefer to play with their own kind. Here's a good link
http://www.cavyspirit.com/sociallife.htm. They are also more likely to try different foods when they have a chum - there's nothing to make a guinea pig try something quite like seeing another piggy eating it and wanting to pinch it from them!