In reply to Alpenglow:
It's a bit chicken-and-egg, you probably need to decide what activities you want to pursue and where before you can assess whether you feel you can manage them by bus with reasonable convenience. There's a regular service up the valley from Modane to Bonneval (roughly every couple of hours IIRC) with a shuttle midi-bus from Termignon up to the southern edge of the National Park. But the summer timetables don't seem to be online yet. The Vanoise Park has published helpful sustainable-transport guides in years past, but again they don't seem to be available yet: http://www.vanoise-parcnational.fr/fr (they used to do an English version of the website but not this year yet ... perhaps another Brexit effect?)
https://shop.thebmc.co.uk/product/mountain-adventures-in-the-maurienne/ gives a lot of useful background info, though it's 12 years old now so may not be quite up to date with public transport.
Personally I'd go further up the valley for a base than Modane, it's a fairly busy industrial/commercial town low in the valley bottom. Termignon's not much more than a village with a picturesque old centre but limited facilities; Lanslebourg is a street town along the valley road, bigger but pleasant.
But the Maurienne's superb mid-alpine country with masses of scope for walking and easy climbing.
