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It compile but I'm not sure this will works correctly

Collect some docs

vsnprintf is ANSI C++99

on cppreference

in case of truncation it returns the total number of characters (not including the terminating null-byte) which would have been written, if the limit was not imposed. Negative value if an error occurred

on VS2013 & VS2015

return the number of characters written if the number of characters to write is less than or equal to count; if the number of characters to write is greater than count, these functions return -1 indicating that output has been truncated. The return value does not include the terminating null, if one is written.

on VS2013

If there is room at the end (that is, if the number of characters to write is less than count), the buffer will be null-terminated.

on VS2015

The vsnprintf function always writes a null terminator...

vsnprintf_s is ANSI C++11

on cppreference

returns number of characters not including the terminating null character (which is always written as long as buffer is not a null pointer and bufsz is not zero and not greater than RSIZE_MAX), which would have been written to buffer if bufsz was ignored, or a negative value if a runtime constraints violation or an encoding error occurred

_vsnprintf_s is MSVC specific maybe be C++11

on VS2013 & VS2015

return the number of characters written, not including the terminating null,... unless count is _TRUNCATE, in which case as much of the string as will fit in buffer is written and -1 returned

on VS2013

If count is _TRUNCATE, then these functions write as much of the string as will fit in buffer while leaving room for a terminating null.

on VS2015(https://msdn.microsoft.com/en-us/library/d3xd30zz.aspx)

writes up to count characters of the given data to the memory pointed to by buffer and appends a terminating null.

Summary

• Return value: On MSVC number of characters written or -1 in case of truncation or error

• Return value: On cppreference returns the total number of characters which would have been written or -1 in case of error

• vsnprintf works as expected on cppreference and VS2015... always writes a null terminator. On VS2013 the buffer will be null-terminated If there is room at the end

My proposl

Use vsnprintf witch is ANSI C++99. On MSVC check for -1 and ensure null terminating on VS2013

static bool cv_snprintf(char* buf, int len, const char* fmt, ...)
{
    va_list va;
    va_start(va, fmt);
    int res = vsnprintf((char *)buf, len, fmt, va);
    va_end(va);
#if defined _MSC_VER
    // maybe truncation maybe error 
    if(res < 0)
        check for last errno 
    res = len -1;
    // ensure null terminating on VS2013	
#if def_MSC_VER<=1800
    buf[res] = 0; 
#endif
#endif
    return res >= 0 && res < len;
}

Still merging snprintf #8769... sure It was only a small oversight.
btw What about the final solution ?