Generate a new vector out of blocks of a given vector

I have a std::vector with roughly 1 million values stored in it. Now I want to divide the vector into N blocks with a given size and create a new std::vector by pulling randomly N blocks out of the original vector. Here is what I have so far, this is just for getting an Idea.

int main {  
    int breakPoint = 2;
    std::vector<int> test = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    std::vector<int> newTest;
    int length = test.size();
    for (size_t i = 0; i < length; i++) {
        int foo = random(breakPoint,length);
        //std::cout << foo << std::endl;
        std::vector<int> subvector(test.begin() + foo, test.begin() + foo + breakPoint);

        for (size_t i = 0; i < subvector.size(); i++){
            newTest.push_back(subvector[i]);
        }
    }
    return 0;
}

int random(int N, int interval){
    int rnd;

    int foo = 1;
    while (foo !=0) {
        rnd = int(randomNumber(0, (interval+1-N)));
        foo = (rnd%N);
    }
    return rnd;
}

randomNumber(a,b) gives an random number in the interval [a,b). This code runs and for not too large vectors I would use it this way. But since I have a large orignial vector and I will have to repeat this new vector operations many times, to gain statistics, I would rather not use this. So my question is, how to make such an operation very fast? The first problem is obviously how I select a breakpoint in random(). Thank you for helping me, cheers!