Vector help, subscript out of range, and basic tutoring

Distant learning student. My lab is to write a function to perform
the addition of large integers, with no limit to the number of digits.
(Also have to do a subtraction, division, and multiplication lab). It
is suggested to treat each number as a sequence. This is what I have
so far, this is rough code just to get it working:

1. I can't get the syntax correct on the 'for' statement in the
longAdditon function. No matter what I try I get a run-time error
subscript out of range.

2. Being very much a rookie, I am sure this isn't the prettiest code;
I am open to ideas, hints, etc. Bear in mind that I am basically self
taught here, and more of a systems admin than a programmer.

#include <iostream>
#include <string>
#include <vector>
#include <sstream>
using namespace std;

int longAdditon(vector<int> &vOne, vector<int> &vTwo);
string toStr(int &i);

int main ()
{
    vector<int> vOne, vTwo;
    string one, two;
    int x = 0;
    size_t r;

    cin>>one;
    r = one.length();

    for (int x = 0; x < r; x++){
        vOne.push_back(int(one[x] - '0'));
    }

    cin>>two;
    r = two.length();

    for (int x = 0; x < r; x++){
        vTwo.push_back(int(two[x] - '0'));
    }

    longAdditon(vOne, vTwo);
}

int longAdditon(vector<int> &vOne, vector<int> &vTwo)
{
    int sum, carryOver = 0;
    string results;

    for(size_t x = vOne.size()-1; x >= 0; x--){
        sum = vOne[x] + vTwo[x] + carryOver;
        if(sum >= 10){
            carryOver = sum - 9;
            sum = 0;
        }
        else{
            carryOver = 0;
        }
        results = results + toStr(sum);
    }
        string::reverse_iterator rit;
        for ( rit=results.rbegin() ; rit < results.rend(); rit++ )
            cout << *rit;

    return 0;
}

string toStr(int &i)
{
    //convert output double to char
    std::string s;
    std::stringstream out;
    out << i;
    s = out.str();
    return s;
}