Using ofMesh indices to establish pairs of vertices

I have an ofMesh object, and I have vertices and indices defined (as well as a way to shuffle my indices), and while in line mode, there are clear “a to b” relationships between these vertices. What I can’t figure out is how to use getVertex to select a and b (and particularly as I’m shuffling indices). I did get it to work in this way:

    int tempFire = ofRandom(numVerts);
    int tempDest = NULL;
        tempFire --;
    tempDest = mesh.getIndex(tempFire+1);
    ofVec3f firingNeuron = mesh.getVertex(tempFire);
    ofVec3f destNeuron = mesh.getVertex(tempDest);
    for (float i=0;i<10;i++){
        float delta = i/10;
        ofVec3f cNeuron = destNeuron-firingNeuron;
        ofVec3f signal = firingNeuron + delta*(cNeuron);

So in that way, a is a random vertex based off of the number of vertices, and b is the vertex with the index one higher. So now I am properly selecting [0,1] [2,3] [4,5], etc. This works well initially, but when I perform this shuffling algorithm:

    //Fisher-Yates Shuffle
    for (int i = numVerts - 1; i >= 0; i --) {
        int index = (int)ofRandom(i);
        int tempIndex = mesh.getIndex(index);
        mesh.setIndex(index, mesh.getIndex(i));
        mesh.setIndex(i, tempIndex);

the “pairings” don’t change with the shuffle.

Would anyone have any advice? If there is any information I’m forgetting to mention, or if I can improve on something, let me know, any help is appreciated.