What is the best way to store unlimited amounts of Dice where each side is a letter inputted by the user?

I am attempting to write some C++ code in which a command allows interactive users to enter 6 letters that will appear each take a side of a dice. There can be an unlimited amount of dice, but there will always be 6 letters, one for each side of the dice. I am currently using a 2-dimensional vector to store the information, however I am not sure if there is a better approach to this. My current approach is giving me a segmentation fault.

            string word;
            string cube;
            vector<vector<char>> v;
            int c = 0;

            cout << "Please enter 6 letters with no spaces." << endl;
            cin >> cube;
            transform(cube.begin(), cube.end(), cube.begin(), ::toupper);
            while(cube.length() != 6){
                cout << cube << " is not 6 letters" << endl;
                cin >> cube;
                transform(cube.begin(), cube.end(), cube.begin(), ::toupper);
            }
            cout << cube << endl;
            for(int i = 0; i < 6; i++){
                v[c][i] = cube.at(i);
            }
            c++;

this is all placed inside a while loop so that the vector can keep being filled.