The reason you get garbage back from the online converters is that they (still) don't handle generics. Try using the trial/demo version of any commercial C# to VB converter instead.

The equivalent VB code is:

VB Code:
  1. Private Private _slaves As List(Of Viewport3D)
  2.  
  3. Public Property Slaves() As List(Of Viewport3D)
  4.    Get
  5.       If _slaves Is Nothing Then
  6.           _slaves = New List(Of Viewport3D)()
  7.       End If
  8.  
  9.           Return _slaves
  10.    End Get
  11.    Set
  12.          _slaves = Value
  13.    End Set
  14. End Property