It's why English nouns lost their masculine and feminine grammatical gender. The people who used both languages had more important things to worry about.
It all points to the case that English is actually a creole language.
Superstratum/substratum theory applies here. English has an Ingvaeonic superstratum and multiple substrata, from Old Norse and Norman French at least, if not more.
I had always chalked up the Viking invasions to "more of the same" after the Anglo-Saxons had settled, but you're of course right that the two (well, three, I guess) groups came from different geographical areas and the Vikings would of course have spoken a distinctly different language.
I guess I conflated the common view that English is originally a Germanic language with French influences with "English is Anglo-Saxon plus French over time".
Effectively, there are three main sources for English:
* The Anglo-Saxons, arriving around the time where the Romans left.
* The Vikings, arriving around 800-1000.
* The Normans, arriving around 1066.
Not that English haven't changed significantly since then, but these three periods represent the largest number of words increases. The other changes have primarily been spelling.