is it OK to walk the Vélodyssée. Is it permitted? Bordeaux to Irun
I don't think it's a case that walking is prohibited, just that the route is predominantly designed for cyclists, and therefore vigilance is required.
Depending on when you are going, such routes are normally busiest at weekends and evenings.
Bordeaux isn't actually on the Veolodyssee and getting on to the route may be a tad tricky on foot.
Not all the Velodyssee is on bike path. Some of it is road, with and without a footpath.
The actual route can zig and zag in such a way that it is less practical to follow religiously on foot.
You may find this site helpful:
Plan a bike route on traffic-free cycleways and quiet roads. Unique mapping, route photos, and free PDF and GPX downloads.
cycle.travel
It is a bike route planning site. It is useful because you can click on Google Streetview (where available) for any point on the route and it favours quiet roads or bikepaths that should be suitable for walking.
I cycled the Velodysee several years ago and recall meeting pilgrims all along the length. Be warned that it can be very, very hot!