2D Vectors: How do I push_back to a specific row/column?

[Inconspicuous]

I am trying to push_back my first value in column 0 and my second value in column 1, however I do not fully understand 2D vector arrays,

int i=0;
vector <vector <string> > my2dArray;
string value1,value2;
for(int i=0;i>20;i++)
{
    cin>>value1;
    cin>>value2;
    my2dArray.at(0).push_back(value1); //Intended to push_back a row in column 0
    my2dArray.at(1).push_back(value2); //Intended to push_back a row in column 1
}
while(my2dArray.at(0).size())
{
    cout<<my2dArray.at(0).at(i);
    cout<<my2dArray.at(1).at(i);
}

Thanks for reading this

[Brenn]

So... What's your problem? I don't see a question... Maybe I'm missing it?

"I do not fully understand 2d vector arrays"
Do you need a tutorial on what 2d vectors are?
Essentially, they're just a vector of vectors.
(Slightly) More clearly: There is a vector. Each element in that vector is a new vector.
That's pretty much it.