class Model
{
public:
    /*  Model Data */
    /...

    //using default constructor

    Mesh createMesh() {
        Mesh mesh;
        meshes.push_back(mesh);
        return mesh;
    }

    void addVertex(Mesh mesh, Vertex v) {
        mesh.addVertex(v);
    }
    void addTriangle(Mesh mesh, Vertex a, Vertex b, Vertex c) {
        mesh.addTriangle(a,b,c);
    }
/...

class Mesh {
public:
    /*  Mesh Data  */
    vector<Vertex> vertices;
    vector<unsigned int> indices;
    /...
// constructor
    Mesh(vector<Vertex> vertices, vector<unsigned int> indices, vector<Texture> textures)
    {
        this->vertices = vertices;
        this->indices = indices;
        this->textures = textures;
        for (Vertex v: vertices) {
            pairings.insert( std::pair<Vertex,unsigned int>(v,count) );
            count++;
        }
        setupMesh();
    }
    Mesh () {

    }

    //function 1
    void addVertex(Vertex vertex) {
        vertices.push_back(vertex);
        pairings.insert( std::pair<Vertex,unsigned int>(vertex,count));
        count++;
    }

    //function 2
    void addTriangle(Vertex a, Vertex b, Vertex c) {
        unsigned int index = pairings[a];
        indices.push_back(index);
        index = pairings[b];
        indices.push_back(index);
        index = pairings[c];
        indices.push_back(index);
        setupMesh();
    }

Model m;
    Mesh mesh = m.createMesh();
    Vertex a;
    a.Position = glm::vec3 (-1,0,0);
    m.addVertex(mesh, a);
    Vertex b;
    b.Position = glm::vec3 (0,1,0);
    m.addVertex(mesh,b);
    Vertex c;
    c.Position = glm::vec3 (1,0,0);
    m.addVertex(mesh,c);
    m.addTriangle(mesh,a,b,c);