C# Definition:

[StructLayout(LayoutKind.Sequential)]
public class KBDLLHOOKSTRUCT
{
    public uint vkCode;
    public uint scanCode;
    public KBDLLHOOKSTRUCTFlags flags;
    public uint time;
    public UIntPtr dwExtraInfo;
}

[Flags]
public enum KBDLLHOOKSTRUCTFlags : uint {
    LLKHF_EXTENDED = 0x01,
    LLKHF_INJECTED = 0x10,
    LLKHF_ALTDOWN = 0x20,
    LLKHF_UP = 0x80,
}

private static IntPtr LowLevelKeyboardProc(int nCode, IntPtr wParam, IntPtr lParam)
{
  if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
  {
    KBDLLHOOKSTRUCT kbd = (KBDLLHOOKSTRUCT) Marshal.PtrToStructure(lParam, typeof(KBDLLHOOKSTRUCT));
    Debug.WriteLine(kbd.vkCode);  // ***** your code here *****
  }
  return CallNextHookEx(_hookID, nCode, wParam, lParam);
}

Visual Basic 9 declaration

    <StructLayout(LayoutKind.Sequential)> _
    Public Structure KBDLLHOOKSTRUCT
        Public vkCode As UInt32
        Public scanCode As UInt32
        Public flags As KBDLLHOOKSTRUCTFlags
        Public time As UInt32
        Public dwExtraInfo As UIntPtr
    End Structure

    <Flags()> _
    Public Enum KBDLLHOOKSTRUCTFlags As UInt32
        LLKHF_EXTENDED = &H1
        LLKHF_INJECTED = &H10
        LLKHF_ALTDOWN = &H20
        LLKHF_UP = &H80
    End Enum

VB Definition:

    <StructLayout(LayoutKind.Sequential)> _
    Public Class KBDLLHOOKSTRUCT
    public vkCode As UInt32
    public scanCode As UInt32
    public flags As UInt32
    public time As UInt32
    public dwExtraInfo As UIntPtr
    End Class

Notes:

This is named "struct" to be consistent with the Windows API name, but it is defined as a class since it is passed as a pointer (AKA reference) in SetWindowsHookEx and CallNextHookEx. Or it can be defined as a struct and passed with ref (ByRef in VB.NET).

http://msdn.microsoft.com/en-us/library/windows/desktop/ms644967(v=vs.85).aspx