How about like this?
Code:
Public Class Form1
    'Base Form
    Protected Overridable Sub ToolStripButton1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ToolStripButton1.Click
        MessageBox.Show("Base Form")
    End Sub
End Class
Code:
Public Class Form2
    Protected Overrides Sub ToolStripButton1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        MessageBox.Show("Form2 Derived Form")
    End Sub
End Class