Adding two numbers as strings from a textfile
Some basic C++ advice:


Don't use using namespace std;. It leads to namespace pollution: it you use another library, you may import names in the global namespace that may clash with those in the std namespace. Anyway, adding std:: in front of the standard library components does not hinder readability.
return 0; at the end of main is useless. If the program reaches the end of main without having encountered any return statement, it will automagically return 0.
You compare signed and unsigned integer values in this line:

for (int i=0; i&lt;line.length(); i++)


That means that you didn't turn on the right compiler flags (-Wall, -pedantic and -Wextra flags that you should almost always use) or that you ignored the warnings. Try never to ignore warnings, they help to write good code. To fix this warning, simply turn int i=0 into std::size_t i=0. Ideally, you should even write typename std::string::size_type i=0 but people generally agree that it is too long, and the main standard library implementations use std::size_t anyway for the length of a string.

The rationale for this warning is that in some cases, the comparison will happen, but small negative integers may be compared to big unsigned integers. On theo ther hand, if you compare unsigned integers with different sizes, you will never have such a problem.
It's really no more than a tidbit, but post-incrementation (++i) may be faster than pre-incrementation (i++). We all know that premature optimization is the root of all evils, but this one is simple enough to write as you go. In other words, you should remember this useful guideline:


  Use ++i if you don't have a specific reason to use i++.

The variable count is never used, you can remove it. You compiler should have told you that it was unused.
Try to organize your headers in alphabetical order:

#include&lt;fstream&gt;
#include&lt;iotream&gt;
#include&lt;string&gt;
#include&lt;vector&gt;


It will help you when you want to check whether a header is already included or not.
If you want to check whether an ifstream was successfully openen, don't use is_open, use operator!:

ifstream myfile("bignum.txt"); //this opens the file bignum.txt
if (not myfile)
{
    // An error occured
    // handle the error
}
// else use myfile

Also, this it not the idiomatic way to read a file line by line:

while (!myfile.eof())
{
    getline(myfile,line);
    // ...
}


Here is how you should do it, std::getline has been designed to be used in such a loop:

while (getline(myfile, line))
{
    // ...
}


Actually, using eof can read past end of file which makes it unsafe to read a file line by line until the end of the file is reached.