Yeah, being able to hold Hispania is an entirely different question. In truth, I have a difficult time imagining a kingdom the size of the Carolingian Empire lasting with their infrastructure. Each region beforehand was its own country, only conquered by Charlemagne and held together by military force. However this is basically a scenario where the Carolingian Empire did stay together, in which case the Franks could throw huge amount of force at Hispania. Now that doesn't mean they'd hold it, but a united Carolingian Empire would certainly be an incredibly powerful military force for the Middle Ages.
While taking Denmark would hurt the vikings, as Denmark seems to have been the most populated of the Scandinavian countries, I'm not so sure it would immediately give the Franks control over the north. Britain is always going the centerpiece of the North Sea, once the vikings are removed. The Baltic Sea is even further from the heartland, and thus unlikely to occupy anything but a peripheral area for the Franks, even with Denmark conquered. And I hope you meant Northeastern America, because I have no idea how reaching the northwestern part of America would work. However areas like the Faroe Islands, Iceland, and Greenland were absolutely necessary for an example of Vinland to ever occur. It's one thing for the vikings to inhabit these places, but I don't think the Franks would give two craps about these cold and dismal islands so far out. Even then however, Vinland was at the very far end of possible travel. It's why the vikings never settled it. Too far and difficult to reach. It's one thing for an isolated ship to reach the area, another to have consistent trade or migration occur. Even then, the Native Americans didn't have anything that would justify establishing such a stretched trade route, as the fur trade was not nearly as important as later times. Even when the Europeans reached America, they didn't care about settling the land. The Spanish and Portugese were focused on gold of the Aztecs, Inca's, etc. To even make a Vinland-Francia trade route, the Franks would need to take England, Iceland, and Greenland. I simply don't see them doing that when Hispania, southern Italy, and the muslims are there presenting closer and more immediate problems/opportunities. The vikings had no real choice as they tried and failed to take the more fertile and populated lands to the south and failed, so these areas were their only real opportunities. For Francia to really focus North beyond England, which already presents a lot of problems, would require quite a few butterflies.
Overall while I could see Francia taking Denmark, I figure it would be a 'shoring up our northern defenses' kind of thing rather than the 'place by which to base a Frankish domination of the North Sea'.