error: invalid user-defined conversion from ‘char’ to ‘std::vector

I'm getting the above error in stl_algo.h and can't pinpoint the problem as I'm a beginner and there are so many vectors involved.

The overall program is meant to solve/complement the app "Just 2 Words": given an allowed word list, a string of allowed letters, and the length of one of the words, find all possible 2-word combinations that can be created from the allowed letters. The details are tedious but below is the problem function.

Excuse the horrible naming and erratic iteration - I'm still trying to learn when it's better or worse to auto-iterate over vectors and how the iterator can or can't be used within the loop (that may be what's causing the error).

vector<string> two_words(vector<string> &subsets, vector<string> &wordlist, string &letters)
{
    vector<string> permutations;

    vector<string> first_words;

    for (auto str: subsets)
        sort(str.begin(), str.end());

    for (unsigned i=0;i < subsets.size(); i++) {
        do {
        permutations.push_back(subsets[i]);}
        while (next_permutation(begin(subsets[i]), end(subsets[i])));
    }


    for (unsigned i = 0; i < permutations.size(); i++) {
            if (find(wordlist.begin(), wordlist.end(), permutations[i]) != wordlist.end())
                first_words.push_back(permutations[i]);

    }

    vector<string> complement_sets;

    vector<string> complement_permutations;

    vector<string> second_words;

    for (auto str: subsets)
        set_difference(str.begin(), str.end(), letters.begin(), letters.end(), back_inserter(complement_sets));


    for (auto str: complement_sets) {
        do {
        complement_permutations.push_back(str);}
        while (next_permutation(begin(str), end(str)));
    }

    for (unsigned i = 0; i < complement_permutations.size(); i++) {
            if (find(wordlist.begin(), wordlist.end(), complement_permutations[i]) != wordlist.end())
                second_words.push_back(permutations[i]);
                }

    vector <string> final_solutions;

    string k;

    for(unsigned i = 0; i < first_words.size(); i++) {
        for(unsigned j = 0; j < second_words.size(); j++) {
           k = first_words[i] + second_words[j];
           final_solutions.push_back(k);
           second_words.erase(second_words.begin()+j);
        }
    }

    return final_solutions;

}

Error: error: invalid user-defined conversion from ‘char’ to ‘std::vector<std::basic_string<char>