DR Malcolm Kendrick had a post some time ago in his long running blog on the causes of heart disease, in which he said that people who have high Lp(a), that is Lipoprotein a, which gives them a much higher risk of cornonary heart disease, might benefit from taking quite high doses of vitamin C.
Lp(a) levels are a genetic thing, it comes from the time in our evolution when foods containing vitamin C might be in short supply and so, when a person cut themselves, as with scurvey caused by vitamin C deficiency, they would bleed very easily. Lp(a) makes blood clots much easier to form so helping those with a vitamin deficiencey in times gone by. Nowadays, of course, we don't usually have a vitamin C deficiency and so Lp(a) is a nuisance as it means those with high levels of it are more likely to get blood clots in their arteries and the trouble that cuases. It's something like that - I can't find the specific blog where Dr Kendrick explained it better than me, but his take home message was that anyone with high Lp(a) (and we don't seem to get that measured in the UK usually) would find no harm in taking extra vitamin C.
To be on the safe side, it doesn't seem to harm anyone to take extra vitamin C. It can upset the tummy but you can buy buffered vitamin C: magnesium ascorbate tablets or powder which doesn't cause a problem.