The Defense stat actually is substantial higher and that does come into play. Vog gear caps at 402 and Crota gear at 447. I think you are confused about how scaling works because even though level doesn't matter above 30 the Defense stats do and that's why a 32 can take much more damage in the vault. I will raid with just about anyone but there is no upside to running vault gear in the vault when there is armor with much better stats out there.