>> wcstombs_s return EILSEQ
For wcstombs() to work on anything other than "C" locale characters (A-Z, a-z, 0-9, etc...), you must call setlocale(LC_CTYPE, "") so that the CRT will convert using the system ANSI codepage (ACP).

Even then, wcstombs() will fail if it encounters any characters that aren't in the ACP. I don't think that "ả" (U+1EA3) or "ị" (U+1ECB) are in any of the Windows codepages, so wcstombs() will never work on windows for those characters.

>> strWText = L"Quảng Trị";
Putting extended characters directly into your source can be dangerous. Anyone who edits/saves that source file will have to save it with a Unicode encoding.

Code:
#include <windows.h>
#include <stdio.h>

HFONT CreateFont(HDC hdc, int pitch, const wchar_t *facename);

const wchar_t *g_wide_string = L"Qu\u1EA3ng Tr\u1ECB";

//------------------------------------------------------------------------------

int main()
{
    const char *className = "ANSI_MsgLoop_Test";
    WNDCLASSA wincl = {0};
    wincl.hInstance = GetModuleHandle(0);
    wincl.lpszClassName = className;
    wincl.lpfnWndProc = DefWindowProcA;
    wincl.style = CS_DBLCLKS;
    wincl.hIcon = LoadIcon(0, IDI_APPLICATION);
    wincl.hCursor = LoadCursor(0, IDC_ARROW);
    wincl.hbrBackground = (HBRUSH)COLOR_BACKGROUND;

    if (!RegisterClassA(&wincl))
        return 0;

    HWND parent;
    parent = CreateWindowA(className, "ANSI Msg Loop", 
                           WS_OVERLAPPEDWINDOW | WS_VISIBLE,
                           CW_USEDEFAULT, CW_USEDEFAULT,
                           300, 100, HWND_DESKTOP, 0, 
                           GetModuleHandle(0), 0);
    if (!parent)
        return 0;

    HWND combo;
    combo = CreateWindowW(L"ComboBox", L"", 
                          WS_CHILD | WS_VISIBLE | WS_DISABLED | CBS_DROPDOWNLIST,
                          10, 10,  260, 30,
                          parent, (HMENU)1, 
                          GetModuleHandle(0), 0);
    if (!combo)
        return 0;

    // give it a nice font
    HDC hdc = GetDC(combo);
    HFONT hfont = CreateFont(hdc, 12, L"Times New Roman");
    ReleaseDC(combo, hdc);
    SendMessageA(combo, WM_SETFONT, (WPARAM)hfont, 0); 

    // add string and select it
    SendMessageW(combo, CB_ADDSTRING, 0, (LPARAM)g_wide_string);
    SendMessageA(combo, CB_SETCURSEL, 0, 0);
    
    MSG msg;
    BOOL bRet;
    for (;;)
    {
        bRet = GetMessageA(&msg, parent, 0, 0);
        if ((bRet == 0) || (bRet == -1))
            break;

        if (!IsWindow(parent) || !IsDialogMessageA(parent, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }//if
    }//while

    DeleteObject(hfont);
    return 0;
}//main

//------------------------------------------------------------------------------

HFONT CreateFont(HDC hdc, int pitch, const wchar_t *facename)
{
    LOGFONTW lf = {0};
    wcscpy_s(lf.lfFaceName, sizeof(lf.lfFaceName)/sizeof(*lf.lfFaceName), 
             facename);
    lf.lfHeight = -MulDiv(pitch, GetDeviceCaps(hdc, LOGPIXELSY), 72);
    lf.lfWeight = FW_NORMAL;
    lf.lfOutPrecision = OUT_TT_ONLY_PRECIS;
    lf.lfClipPrecision = CLIP_STROKE_PRECIS;
    lf.lfQuality = CLEARTYPE_QUALITY;

    return CreateFontIndirectW(&lf);
}//CreateFont
Notes:
- No extended characters in source (save source as Unicode otherwise)
- ComboBox created with CreateWindowW()
- Created a nice font that should have support for those Unicode characters (was getting "best fit" characters without it)

gg