Hi,
Unfortunately, at this point you can't.
The reason is that VB doesn't support lambda expessions, and as for anonymous delegates, it doesn't support Action(Of T). So while you can use th AddressOf operator instead of the lambda, you can do this for expressions that return a value, not void ones. This is a language issue that will be resolved in VB 10.
But until the, you can use Natural or Reflective mocks. I can tell you that we are in the works of fitting the features of AAA to VB syntax. We'll keep you updated as we continue.
Sorry for the inconvenience,