C++ :: Sorting On Two-dimensional Vector

Suppose I have a two-dimensional stl vector (a vector of vectors), for example:

Code : vector<vector<int> > x;

And then I want to sort the outer vectors in order of the size of their inner vectors. How do I do this?

Usually, with a one-dimensional vector, I can just create a comparison function and use the sort function. For example, if my vector is defined as:

Code:
vector<int> y;

And I want to sort if in terms of the int values of each vector element, then I can sort by using the stl::sort function, with the comparison function defined as:

Code:
static bool SortFunction(int val1, int val2) {
if (val1 > val2) {
return true;
} else {
return false;
}
};

However, with my two-dimensional vector, I use the following comparison function:

Code:
static bool SortFunction(vector<int> vec1, vector<int> vec2) {
if (vector1.size() > vector2.size()) {
return true;
} else {
return false;
}
};

I get a runtime error:

"Debug Assertion Failed!
File: c:Program Files (x86)Microsoft Visual Studio 10.0VCincludealgorithm
Line 3686
Expression: invalid operator<"

Why is this?