I imagine that the tumour would quickly evolve to repress expression, mutate, or completely delete the targetted neoantigen as the neoantigen is probably not required for viability.
This is absolutely true, and actually the mutations are usually quite diverse already, so it's thought that multiple neoantigens should be targeted simultaneously. However, no matter how much you target, the diversity and evolution of cancer always wins. Therefore, the goal is not to target every last cancer cell, but to trigger enough of an immune response across a wide enough variety of mutations for the B cells to adapt to whatever else they find. This is also the theory of checkpoint blockades, and they will probably be used together.
An individualized vaccine for cancer has to be created and administered fast, within the course of a specific patient’s cancer treatment.