The EXTENSION_VECTOR structure encapsulates a variable-length array of EXTENSION structures (section 2.2.5.1.4) that are stored consecutively. The EXTENSION_VECTOR structure is used in the Extensions field in the NEGO_MESSAGE structure, as specified in section 2.2.6.3. 

 struct
 {
   ULONG ExtensionArrayOffset;
   USHORT ExtensionCount;
 } EXTENSION_VECTOR; 

ExtensionArrayOffset: A ULONG type array. Each element contains an EXTENSION structure, as specified in section 2.2.5.1.4.  

ExtensionCount: A USHORT that contains the count of elements in the ExtensionArrayOffset field.