Using Unmanaged C++ DLL in VB.NET With Pointers

I have source written in C++ as an example that I'm trying to port to VB.NET.  It uses a 3rd party DLL that I do not have the source from.

This is the declaration for the function in C++
char* rcg(const char *src_file, char* return_buf, int size_retbuf);

Here is my declaration
 Private Declare Function rcg Lib "rcg.dll" Alias "rcg" (ByVal src_file As String, ByVal return_buf As IntPtr, ByVal size_returnbuf As Integer) As IntPtr

This is a snippet of the code I am trying to duplicate in C++
      #define RETURN_BUFFER_SIZE 24
        char lpszFileName[] = "r.rcg";
      char lpszReturnBuffer[RETURN_BUFFER_SIZE] = {0};
        rcg( lpszFileName, lpszReturnBuffer, RETURN_BUFFER_SIZE );

And after a lot of stuff that didn't work I have this in VB.net
        Dim d As IntPtr = Marshal.AllocCoTaskMem(24)
        Dim result As IntPtr = rcg("r.rcg", d, 24)

I am trying to output the result of d in my code (the lpszReturnBuffer in the C++).  I'm not sure how to copy that memory into a byte array or string that I can work with.  I do realize that I am perhaps a bit over my head but I'm trying hard to learn here.  I have been changing this around for about 24 hours and figured it was time to ask the experts!