[Йоха]

вот так 100% верно:

void logcustom(uint8 lvl, Color color, const char *str, ...)
{
	char buf[4096]; //

    if(!str || loglevel < lvl)
        return;
    va_list ap;
    _log_setcolor(true,color);
    if(logtime)
        printf("%s ", GetTimeString().c_str());
    va_start(ap, str);
    vsprintf(buf, str, ap );
    va_end(ap);
	char* str_oem = str_convert_log(buf, CP_UTF8, 866);
	printf("%s\n", str_oem);
    _log_resetcolor(true);

    if(logfile)
    {
        fprintf(logfile, "%s %s\n", getDateString().c_str(), str_oem);
        fflush(logfile);
    }
    fflush(stdout);
	delete[] str_oem;
}