Came here to ask this exact question.
As I understand it, Observable Plot also seeks to be the "higher-level abstractions on top of D3" layer.
The Vega docs address Vega vs D3 (https://vega.github.io/vega/about/vega-and-d3/), but I don't see them compare Vega vs Observable Plot, which would seem to be a more apples-to-apples comparison.