Was all of Paris on those two islands in the Siene River when the Vikings attacked?
Scroll down a little, and there’s a map of Paris in the era, with the note that the city was concentrated on Ile de la Cite.
However, there likely were some outer towns and villages, in a patchwork of farms, as this was the usual pattern. So “All” of Paris…well, how do you define it? It’s a “fuzzy boundary” problem.
The Vikings set siege to the town, and that suggests that they pretty much wiped out any outer towns and villages. That, too, was a convention of the era: often the defenders themselves would set fire to houses outside city walls, to clear a field of fire and to deny the enemy timber to use in siege engineering.
Well the Ile de la Cité was where the old Roman walls stood so, yeah, once those were fixed up the island became the safe refuge for everyone around - prior to that, between 845-870 the Vikings would come and the population would just flee en masse because it simply wasn’t defensible against river-borne raiders.
There were plenty of little burghs and abbeys around the island on both sides of the river though, many of which became part of the city proper in later centuries - Saint-Germain-des-Prés, Saint-Denis, Sainte-Geneviève, Saint-Cloud… those weren’t Paris per se but its inhabitants were under the protection of the Counts of Paris all the same.