Next trick. Let's force it to be defined.
Compile this and Link this before any other library and see what happens.
library vp;
// For WIN64 only, otherwise cdecl
uses unixtype;
type
Tvsnprintf = function(s:Pchar; n:size_t; const format{array of char}):integer;varargs;stdcall; //win64 has no cdecl!, just stdcall (or a winapi macro to stdcall)
var
__ms_vsnprintf:Tvsnprintf = nil;export;
begin
end.
That should solve the linker issue, but of course the function can not be called.
I suspect it is in msvcrt.dll or in the proprietary safestring library. by MS. (That's not a dll but a static library, it comes with the SDK, however and also with all versions of visual studio including the community edition)