As a brief history lesson:
Late medieval is the 14th and 15th centuries. The first use of firearms in a European warfare date to the 13th century. From a military standpoint (in Europe) the late medieval period is when firearms were first finding their place in warfare.
In the real world, the vikings were mostly gone by the 1100s. So they didn’t last to see the rise of firearms or the late medieval period. But, if you’re using piratical raiders, in a fantasy setting
based on the late medieval era, then guns are a rapidly growing part of
During the 14th century, gunpowder weapons were used primarily as siege equipment, gradually giving way to use in infantry combat as the technology was refined.
The 14th century started with a sudden climate shift in Europe called The Little Ice Age (this lasted into the mid-19th century, if I remember correctly), which strained Europe’s agriculture base. Simultaneously, Europe’s population had been gradually shifting into cities. The densely packed urban areas created ideal circumstances for the spread of disease. Specifically, Bubonic Plague, which wiped out over half of Europe’s population in under a century.
The Little Ice Age is particularly interesting, if you’re working with pseudo-vikings, because it caused in seasonal freezing of many rivers and straits in Europe. This resulted in battles where infantry was marched across the ice, to assault cities that, traditionally, would have required a naval force, and simultaneously, left the defender’s ships frozen in harbor.
The cold also resulted in a serious population decline in Northern Europe, due to famine, the arctic conditions, and (I assume) migration.
That might give you some ideas to play with in working out the details of your setting.
In the real world, the Vikings used chain mail (only one surviving shirt has ever been found) and lamellar armor (small metal reinforced plates, bound together in a grid pattern). Leather garments have survived, and they were probably intended for use as armor.
For weapons, the Vikings used swords (rarely), axes, spears, knives (called “knifr” or “seax” depending on the design), and bows. Spears were used both as thrown weapons and in close quarters. Bows, as I understand it, were used for both hunting and warfare. I’m not sure if they had distinct designs for each. I’m also, almost certain, the vikings didn’t use composite bows. While those already existed, the glues that held them together would have been water soluble, making their use at sea, “extremely problematic.” Swords were time consuming to make, making them rare and expensive.
It’s probably worth pointing out, with the Vikings, the best insight into what they used as arms and armor come from their burial sites. This means if something was too valuable to use as grave goods, or was given to an heir (such as swords), it’s probably disproportionately rare. Also, from what we know, most of the hilts would have been fashioned out of organic materials (like bone, ivory, and wood) which didn’t manage to survive the thousand years it took for archaeologists to find them. The same goes for any cloth armor they may have used. (It’s a bit of an open question.)
Moving forward a couple centuries, probably would have resulted in higher quality swords that were more easily produced. Meaning they could be longer, and more numerous. They would also probably retain most of their desirability from when they were rare, though their value as status symbols would suffer.
I don’t know what changes Viking Armor would have seen, moving forward. Though, it’s worth pointing out the Vikings did frequently take foreign weapons and armor as plunder. So whatever armor (and weapons, for that matter) they’re using, would probably be either patterned off foreign designs, or outright stolen.
The thing I can’t address at all is how the Viking ship design would have changed and evolved. I just don’t have the background to speculate intelligently on that subject, sorry.
I hope that gives you some ideas to work with, at least. It might not be the setting you had in mind, but there’s certainly a potential setting to play with there.