> Why would a variant which has apparently evolved to evade the vaccine continue to react to prior vaccination?
This is over-simplified. In particular, "react to prior vaccination" is conflating a bunch of different things, involving antibodies, B cells, T cells, etc. My understanding is that (1) such data as we have seems to generally point toward protection-from-infection being badly eroded but protection-from-illness/death being only moderately eroded, (2) if you're boosted, protection is less badly eroded, (3) all of this seems to fit with our general understanding of how the immune system works. I don't have the domain knowledge to unpack that further.