Yes it needs to be removed to be re enamelled and there are only a handful of companies that do it. We looked into it and decided against as I'm not sure we could get it out! There are other options but I think you have to be careful as some of the resurfacing doesn't last too long before wearing and needing redoing.
That link though isn't for vitreous enamel, it's resurfacing which has very mixed reviews when you research it. I assume the vitreous enamel process to be hugely more expensive as it involves so much more but if looked after properly will last a lifetime.This is the link I was looking for.www.ive.org.uk/map/index.html As I said we decided against due to logistics but the resurfacing options will last 10 years max. We've found a local man to do ours and he is a real enthusiast but he was very straight with us and said 10 years. It's going to cost around £300. Hope that helps.
We had ours done a few years ago and it was brilliant and didn't need removing. My dad his avocado suite re-enamelled white and it worked wonderfully, again it was done in situ. Cost a couple of hundred quid from memory.