The frankish king gave him Normandy because Rollo was encline to accept frankish suzerainity and to defend the Seine's mouth against other Vikings.
Without this feudalisation of Rollo's troops, I doubt they would have been able to take Normandy even the 1/3 of the region that was settled by Scandinavians after 911.
To resume, they didn't gave them west Neustria because they conquered or could have conquered them, but because it was the perfect place to put a march against other raiders. It was the last of "foedus" in western Europe if you want.
Besides, if vikings were a threat, it wasn't a threat of "controlling and/or razing everything", it was the usual piracy threat not a conquering one, because Vikings raids were far from having enough men to controlling some region in the continent (Northern England is an exception, due to the very low population, and when they tried to do the same with Murcia and East-Anglia, they failed to).
For the power vaacum, you quite well described the OTL situation. During the 4th siege of Paris by exemple, the defense wasn't planned by the power in place, but by the inhabitants and local authorities without help worth of mention coming from outside.