Mary Roach (2013) discusses the diet of the Inuit at length in her book Gulp. Because fruits and vegetables are hard to come by where the Inuits live, they make do with organ meats from the animals they slaughter. In the book, she states that some organ meats (especially those of carnivores') are especially rich in vitamins (e.g. a cow's lung contains more vitamin C than an orange does).

Scurvy wasn't as much of an issue for the northern natives simply because vitamin c is actually present in a wide variety of food products, especially when they are eaten fresh.

Meats, birds, fish and blubber (both from whales and seals) are stocked with vitamins. Also you can get your daily vitamin c intake from pine needles (brewed in a tea for instance). Though pines aren't common everywhere in the north they are still very widespread. Interestingly enough the most famous scurvy issue in the north - during the Yukon gold rush - could have been easily avoided if the prospectors had listened to the locals.

Unfortunately that didn't happen, during the end of the 19th century most Americans and Canadians viewed first nations as primitive barbarians and could not accept the idea that they had knowledge the white man didn't. So prospectors tried to rely on the vitamin sources they knew : citrus, vegetables and potato skin (eaten raw, very vitamin intensive and cheap). However with winter snows the passes to Whitehorse and Dawson City closed, and these produce were hard to come by (a steak would cost quite a few gold nuggets, and an orange was impossible to find).

So all in all the traditional fare was quite sufficient for first nations to get their vitamins, and if you look at the current food issues up there they are often linked to the population moving away from their traditional sources of food - whether it is because the animals have slowly disappeared (due to pollution or over hunting), because the population has grown beyong sustainability on local production, or simply because the knowledge was lost with the death of elders.

Edit: this mainly concerns scurvy and vitamin c. There are other vitamins which have always been a problem, but this isn't limited to northern populations in canada.