I'm no anthropologist, but I'm pretty sure it takes longer than half a century for a civilization to fall and be rebuilt as a mystical tribe using spears and herbs for survival.
Why, what else did their founders have? Not a lot. Just what they could carry. No Geck, no Cache of Guns (and no skills to maintain the guns they had or make bullets), no Chemical labs to build complicated medicens
Considering Tandi is still alive since the first Fallout story, the whole tribe story makes no sense at all. At best it would have been 80 or so years, how does one even manage to revert back to such a primitive civilization of ethnic looking people? considering that the Valutdweller built a fairly modernized civilization to begin with.
Considering that the Life expentancy is a lot lower in those conditions (think 40-ish like some african countries today), you're looking at their 80 years not really being comparable to our 80 years - its enough time for people born on day one to be born, have kids, die, and for their Kids to die.
And what was with adding magic into the story (the magic dream communication)? kinda ruined the sci-fi aspect of it for me.
Psionics (not magic) are a common Sci Fi Element, and were present in FO1.