How to "move" Eigen::VectorXd s

A commenter in a recent post of mine told me I need to utilize c++11 move-semantics better to deal with a bottleneck in my code. Below is a simplified version of what needs to be fixed.

Question: inside the for loop of makeCopy, oldV[i]is an lvalue, so how could I do something like mandatoryCopy[i]&& = oldV[i]? This is the primary bottleneck, right? I'm thinking something like mandatoryCopy[i]&& = std::move(oldV[i]), but this obviously won't compile.

#include <iostream>
#include <Eigen/Dense>
#include <vector>

void makeCopy(std::vector<Eigen::VectorXd> &oldV){
    int n = oldV.size();
    std::vector<Eigen::VectorXd> mandatoryCopy;
    mandatoryCopy.resize(n);

    for(int i = 0; i < n; i++){
        mandatoryCopy[i] = oldV[i];
    }

    // swap the two
    oldV = mandatoryCopy;
}

int main(int argc, char **argv)
{
    // starting vector
    int len(1000);
    Eigen::VectorXd placeHolder(50);
    std::vector<Eigen::VectorXd> v(len, placeHolder);

    // copy it a bunch of times
    for(int iter = 0; iter < 1000; ++iter){
        std::cout << "iter: " << iter << "\n";
        makeCopy(v);
    }

    return 0;
}