The "part" of the virus targeted by the vaccine is the spike protein which is "The spike (S) protein of SARS-CoV-2, which plays a key role in the receptor recognition and cell membrane fusion process" [1].

A mutation which alters the shape of the spike protein, rendering it protected against the vaccine will most likely render the virus less virulent.

Think of it as a hedged bet.

https://www.nature.com/articles/s41401-020-0485-4