Use Blend From Shape:
- Select all vertices whose positions you want to reset,
- Execute Blend From Shape, accessible from W, Ctrl-V or Mesh > Vertices menu,
- Set "Basis" as shape key to blend from (by default),
- Set blend factor to the maximum value 1.0 (by default),
- Uncheck Add, so only values from "Basis" will be blended in, effectively reverting all selected vertices' position.
Kind of wish there was a slightly less arseabout way to do it but a way is better than no way.