I have now expanded the vertexNumbersHelper to BufferGeometry.
Since non-indexed BufferGeometry has no vertices / indices, I use the internal array vertexPositions.
The triangles involved are stored in this array for each defined vertex. More specifically, the positions of the respective corners.
So I take the first entry for every vertex n.
vertexNumbers[ n ].position.set( … .vertexPositions[ n ][ 0 ] …
At http://sandbox.threejs.hofk.de/ the new possibility is now integrated.
The extended version is also available on GitHub GitHub - hofk/THREEf.js at master