Init std::string with single copy

I have the following code in C++ on Win32. It's simply a C++ warp on some Win32 API that returns "CHAR *"

wstring expandEnvironmentVariables(const wstring & str)
{
    DWORD neededSize = ExpandEnvironmentStrings(str.c_str(), nullptr, 0);
    vector<WCHAR> expandedStr(neededSize);
    if (0 == ExpandEnvironmentStrings(str.c_str(), expandedStr.data(), static_cast<DWORD>(expandedStr.size()))) {
        return wstring(str);
    }
    return wstring(expandedStr.data());
}

What bothers me about this code, is the double copy of the result.

1. by the API into a vector of WCHARs.
2. from the vector into std::wstring.

Is there a way to implement this code with just a single copy, and without a major change to the signature of the function. This is a specific example, but I'm more interested in the general solution and the right way to work with std::wstring / std::string, because this pattern shows itself in many places in the code.