Spike proteins in coronaviruses are present in two structurally distinct conformations: pre- and postfusion. The conformational switch between them promotes the viral fusion to the host cell.
To maximize the effectiveness of the vaccine, its spike protein should stay in the prefusion state no matter what. The two mutations in the engineered spike proteins allow exactly that.
More or less so. Roughly speaking, the effectiveness of the vaccine should be the same as long as the shape of the spike protein doesn’t change.
The most important mutation in the new VOC 202012/01 variant (N501T) translates to tighter binding to the ACE2 receptor, which should have no effect on its structure. However, that variant has also a double deletion (residues 69 and 70 are removed) that might induce slight conformational changes.
Another thing to keep in mind is that vaccines are polyclonal: the antibodies produced by them target several different regions of the spike protein. This implies that the virus would need to mutate several times in separate parts to be able to evade immunity effectively.
To maximize the effectiveness of the vaccine, its spike protein should stay in the prefusion state no matter what. The two mutations in the engineered spike proteins allow exactly that.