■No33349に追記(魔界の仮面弁士の記事)
> なお、質問内容からは外れてしまいますが、もしもやりたいことが
> {a5cd92ff-29be-454c-8d04-d82879fb3f1b} の IVirtualDesktopManager を
> 利用するという点だけなのであれば、C# を介さずに、
> VB6 や VBA から直接呼び出すという選択肢もあるかも。


試しに作ってみました。Form1 からであれば
　Dim c As VirtualDesktopManager
　Set c = New VirtualDesktopManager
　If c.IsVirtualDesktop(Me.hWnd) Then
　　MsgBox "OK"
　End If
のように利用できるはずです。
（手元に VB6 が無いので、動作検証は 32bit 版の Excel 2016 で行っています）


'-----------------------------------------------------------
'クラスモジュール "VirtualDesktopManager"
Option Explicit

Private oVirtualDesktopManager As IUnknown
Private pIVirtualDesktopManager As Long

Private Declare Function IIDFromString Lib "ole32" _
   (ByVal lpsz As Long, _
    ByRef lpiid As Byte) As Long

Private Declare Function StringFromGUID2 Lib "ole32" _
   (ByVal rguid As Long, _
    ByVal lpstrClsId As Long, _
    ByVal cbMax As Integer) As Long

Private Declare Function DispCallFunc Lib "oleaut32" _
   (ByVal pvInstance As Long, _
    ByVal oVft As Long, _
    ByVal cc As Integer, _
    ByVal vtReturn As Integer, _
    ByVal cActuals As Long, _
    ByRef prgvt As Integer, _
    ByRef prgpvarg As Long, _
    ByRef pvargResult As Variant _
) As Long


Private Sub Class_Initialize()
    Dim iid(0 To 15) As Byte
    Call IIDFromString(StrPtr("{a5cd92ff-29be-454c-8d04-d82879fb3f1b}"), iid(0))
    Set oVirtualDesktopManager = GetObject("new:aa509086-5ca9-4c25-8f95-589d3c07b48a")
    Call Invoke32(ObjPtr(oVirtualDesktopManager), 0, VarPtr(iid(0)), VarPtr(pIVirtualDesktopManager))
End Sub

Private Sub Class_Terminate()
    Call Invoke32(pIVirtualDesktopManager, 2)
    pIVirtualDesktopManager = 0
    Set oVirtualDesktopManager = Nothing
End Sub

Private Function Invoke32(ByVal pInstance As Long, ByVal fIdx As Long, ParamArray args() As Variant) As Variant
    Dim l As Long, u As Long, c As Long
    l = LBound(args)
    u = UBound(args)
    c = u - l + 1

    Dim vp() As Long
    Dim vt() As Integer
    ReDim vp(IIf(c = 0, l, u))
    ReDim vt(IIf(c = 0, l, u))

    Dim i As Long
    For i = l To u
        vt(i) = VarType(args(i))
        vp(i) = VarPtr(args(i))
    Next

    Dim hr As Long
    hr = DispCallFunc(pInstance, fIdx * 4, 4, vbLong, c, vt(l), vp(l), Invoke32)
    If hr <> 0 Then
        Err.Raise hr
    End If
End Function

Public Function IsWindowOnCurrentVirtualDesktop(ByVal topLevelWindow As Long) As Boolean
    Dim outBool As Long
    Dim hr As Long
    hr = CLng(Invoke32(pIVirtualDesktopManager, 3, (topLevelWindow), outBool))
    If hr = 0 Then
        IsWindowOnCurrentVirtualDesktop = CBool(outBool)
    Else
        Err.Raise hr
    End If
End Function

Public Function GetWindowDesktopId(ByVal topLevelWindow As Long) As String
    Dim hr As Long
    Dim iid() As Byte
    ReDim iid(0 To 15)
    hr = CLng(Invoke32(pIVirtualDesktopManager, 4, (topLevelWindow), iid(0)))
    If hr = 0 Then
        Dim bIID() As Byte
        bIID = String$(40, 0)
        Call StringFromGUID2(VarPtr(iid(0)), VarPtr(bIID(0)), 40)
        GetWindowDesktopId = Left$(UCase(bIID), 38)
    Else
        Err.Raise hr
    End If
End Function

Public Sub MoveWindowToDesktop(ByVal topLevelWindow As Long, ByVal desktopId As String)
    Dim iid(0 To 15) As Byte
    Call IIDFromString(StrPtr(desktopId), iid(0))
    Dim hr As Long
    hr = CLng(Invoke32(pIVirtualDesktopManager, 5, (topLevelWindow), iid(0)))
    If hr <> 0 Then
        Err.Raise hr
    End If
End Sub