I did it the same way, with a minor performance improvement: I arrange v1 and v2 so that I know which is larger:

node* lca_(node* root, int a, int b)
{
    if (b < root->data) {
        return lca_(root->left, a, b);
    }
    else if (a > root->data) {
        return lca_(root->right, a, b);
    }
    return root;
}

node* lca(node* root, int a, int b)
{
    if (a < b) {
        return lca_(root, a, b);
    }
    return lca_(root, b, a);
}