Pellets are basically designed to be a complete feed and incorporate everything a guinea pig needs if they're not given anything else. They don't of course as hay is essential to teeth and digestive health, and there's no substitute for getting all the vitmanins and minerals you need from fresh sources (never mind the huge boredom factor from only eating pellets). If you feed your guinea pig a good quality, fresh and varied diet then they should get everything they need from that and actually not need pellets. Many vets agree with this. But some people like to give a few anyway, just as a back-up to make sure their diet is balanced, or simply to add a bit of variety.
I feed mine a good quality hay, readigrass (dried grass) and a good variety of fresh vegetables every day, with occasional fruits thrown in. They get pellets too, but to be honest they don't really eat that many. They ate a lot when I first got them but now eat more hay, readigrass and vegetables than pellets and I'm happy with that. I haven't ever wanted to remove them completely though, as they enjoy a few and they are good quality pellets so I kind of see them as a fall back if they haven't eaten as much of something else that day.
Have a look at this list of vitamin C content in veggies
https://www.guinealynx.info/nutrition.html#order-c and make sure you're giving veggies that are a good source. It's definitely possible to provide all they need from fresh sources. I only ever supplement with vitamin C when a pig is ill when they need a little extra.
