C++ variable arguments with std::string only

Question

I'm trying to create a function that takes a variable amount of std::string arguments and formats a string with it.

Example:

Test::formatLine(const string::format, ...)
{
    const std::string buffer;
va_list args;
va_start(args, format);
vsprintf(buffer.c_str, format.c_str, args);
va_end(args);
cout << buffer << endl;
}

Compiling this snippet errors:

Error   1   error C3867: 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>::c_str': function call missing argument list; use '&std::basic_string<char,std::char_traits<char>,std::allocator<char>>::c_str' to create a pointer to member

What I want to achieve:

Test t = Test();
t.formatLine("Hello %s!", "monsieur");

should print Hello monsieur!

t.formatLine("Hello %s %s! How %s you today?", "good", "sir", "are");

should print Hello good sir! How are you today?

Is it even possible to use va_list and vsprintf with std::string only, avoiding char buffer[size]?

Working example (so far) with fixes suggested by Igor, using buffer:

void Test::formatLine(string format, ...)
{
    char buffer[256];
    va_list args;
    va_start(args, format);
    vsprintf_s(buffer, format.c_str(), args);
    va_end(args);
    cout << buffer << endl;
}

Using Igor Tandetnik's suggestion and sample code I finally got a working example that does not use char buffer[size]:

void Test::formatLine(string format, ...)
{
    vector<char> buf(256);
    va_list args;
    va_start(args, format);
    vsnprintf_s(&buf[0], buf.size(), buf.size() + strlen(format.c_str()), format.c_str(), args);
    va_end(args);
    cout << &buf[0] << endl;
}

Answer 1

A Production Quality Answer

#include <cstdarg>
#include <string>
#include <vector>

// requires at least C++11
const std::string vFormat(const std::string sFormat, ...) {

    const char * const zcFormat = sFormat.c_str();

    // initialize use of the variable argument array
    va_list vaArgs;
    va_start(vaArgs, sFormat);

    // reliably acquire the size from a copy of
    // the variable argument array
    // and a functionally reliable call
    // to mock the formatting
    va_list vaCopy;
    va_copy(vaCopy, vaArgs);
    const int iLen = std::vsnprintf(NULL, 0, zcFormat, vaCopy);
    va_end(vaCopy);

    // return a formatted string without
    // risking memory mismanagement
    // and without assuming any compiler
    // or platform specific behavior
    std::vector<char> zc(iLen + 1);
    std::vsnprintf(zc.data(), zc.size(), zcFormat, vaArgs);
    va_end(vaArgs);
    return std::string(zc.data(), zc.size()); } 

#include <ctime>
#include <iostream>
#include <iomanip>

// demonstration of use
int main() { 

    std::time_t t = std::time(nullptr);
    int i1 = 11; int i2 = 22; int i3 = 33;
    std::cerr
        << std::put_time(std::localtime(& t), "%D %T")
        << vFormat(" [%s]: %s {i1=%d, i2=%d, i3=%d}",
                "DEBUG",
                "Xyz failed",
                i1, i2, i3)
        << std::endl;
    return 0; }