Okay. Viking means men of the Viks, the fjords. Basically they were farmers and traders who got a taste for raiding.
Raiding not only got them goods to trade, but it got them slaves. Slaves they traded throughout the Viking world, through the Mediterranean, the Black Sea, down through into the near east.
Why did they suddenly do this ?
Economics.
They had finally effectively accessed the iron ores of the Norwegian mountains and exploited them to great effect. That iron meant that they could have iron shod ploughs. Those ploughs opened up more land, even poor land, and allowed them to grow more crops. Those extra crops fed more children to healthy adulthood. Those children were married in their teens and started producing their own children.....and within two generations they had a population explosion.
Those children wanted farms and goods of their own. The extra womenfolks meant more wool could be spun (five spinners needed to keep one handloom weaver in production) that meant more good cloth, cloth that became sails as well as clothing.
The longships were the method of the Viking expansion, not the reason for it.
Food wise you have to think seasonality. Their diets were very varied and healthy.
Grains, greenery, roots and fruits. Meat, milk, fish, fowl and eggs. Most animals were slaughtered in late Autumn, since it wasn't possible to feed them all through the Winter. Cows, goats and sheep go 'dry' then anyway and farmyard fowl stop laying eggs. Butter and cheese are good to extend the calcium and calorie rich foods though.
Fish is always available, and it preserves well; dried, salted, pickled or smoked, and they knew all those methods.
Grain means not only ale and beer but porage and bread. If you can brew beer you have barm. Barm is a frothy yeast. That frothy yeast means that bread need not be a wee hard brick
It would be wholegrain though, well unless some poor female slaved away making sure you only got the white
It takes a lot of grinding to make flour so it's not wasted. An iron plate, a griddle/girdle/greddle bakes good bannocks without the need for an oven. The Vikings are generally considered to be aceramic, they used soapstone instead of pottery, but those pots if properly positioned in a hearth and covered bake good bread too. (Look up plant pot loaves for modern examples.)
They didn't have sugar, but they did have honey, and fruits; lots and lots of fruits. Everything from apples and pears to plums and cherries. They also exploited rosehips, cloudberries, lingonberries, etc., etc.,
Seasoning's a little different, but herbs are ubiquitous, and there are loads of native plants that give good strong flavours. Horseradish, cress, mustard and the like.
There was also the range of wildfowl, wild boar, deer, seal, and whale meats available as well as the shellfish.
Generally what we would consider a hard life, but it wasn't unhealthy.
cheers,
Toddy