That's what I mean. For them to be more than passing conquerors they need to step up their immigration to conquered areas. The task was to make them more successful and I think that really the only way to do that is to pump up their populations so that they don't integrate.
First, even if they managed to empty out all Scandinavia, they wouldn't have enough people to outnumber local populations (maybe at the exception of British Isles).
Hedeby, that was one of the most important (demographically speaking) place inhabited by Scandinavians didn't went much more than some thousands (the town being inhabited by other populations than Norses : East Slavs, Germans, etc)
Now, you don't get the goal of norse colonization : it wasn't to fill up the land with Vikings, but to get territory for different reasons.
1) Controlling the region : usually garrisoning an island or coastal place during winter.
It allowed them to ransom and racket the region rather than plundering it, but as well to serve as trade post.
Such presence was quite contradictory with "civilian" settlement as it was precarious (both military and demographically, being dominantly male population)
This was, by far, the most common viking presence in Europe and if plunders and "hit and run" raids are kept as the norm, or longer than IOTL, such presence would be less important to begin with.
Summarizing it : Vikings raids weren't made by farmers in need of lands, but by warriors in need of much wealth.
2) Holding the region.
It's really more rare, and usually led by people that can't reallistically return home to enjoy a better status (as exiled leader). It's the case for Northern Atlantic colonisations (Island, Greenland) and for Eastern England and Normandy.
In these settings, leaders actually tried to attract a population they were used to (including by familiarity with "organisation" customs as for agriculture solskifte and/or tof), trying to control more easily the land by more strong loyalty ties, but also as a mean of prevention against Norse raids.
In this
map of Normandy settlements (the degree of settlement was speculated from toponimy), you can see that they seems to have been largely restrained to coasts and navigable rivers' mouths. After all, one of the responsabilities of Rollo and his descendents, was to keep off possible raiders.
It's also telling that the a good part of settlers seems to have come, not directly from Scandinavia, but from England when Anglo-Saxons pushed back Anglo-Norses (and we know that Rollo went to England several times before). Some most probably came from Northern Europe still, but hardly in large numbers.
Taking away people was harming the local kings or petty kings power. if it helps : "Why of course I'll allow you to take entieres village of people I rule, because I'm that generous and not at all cautious about giving away part of my power".
In short, even discarding the issue about scandinavian human ressources, how Norse presence established itself prevent any real tentative to take-over demographically the regions they controlled (would it be only for that the said region would be still worth controlling).
Finally, there's the cultural issue.
Not only Norses didn't "culturally" took-over, but they adopted quickly local customs.
The only exception worth of mention I can think of is about the legal part : Danelaw, custom of Normandy, by exemple.
The diversity of large settlements (Anglo-Norses, Danes, Norwegians, etc.) prevented the long term maintain of a "pure" norse culture in the conquered lands, with the appearance at best of creole cultures (Anglo-Norse, Norse-Gaël), or absorbtion within the background culture (while, of course, specific traits were maintained).
In fact, the cultural "appeal" of christian cultures could be really important.
For the norse elites (the germanic rulership provided less power, and less stability in the rule than feudal or even vassalic one), but as well for the overall settlers (as a settler, you generally try to get along the locals, in order to not have your farm burnt to the stake).
Baptism being generally a condition sine qua non for the acceptance of settlement (whatever they were genuine or not, that's irrelevant), that provided certainly a way to fusion of populations (remember that, in this period, religious background was far more decisive than cultural) that was obviously at the "benefit" of the overall dominant culture (that benefitted also from being concieved as a "christian" culture, at the difference of norse that wouldn't be considered as such before the XII, if not XIII).
Quoting a contemporary account
Disdaining their own inheritence, hoping acquiring one greater, only starving for gains and domination, tending to imitate all things, standing in the good half of prodigality and greed, having doubtlessly managed to unite values apparently opposed.