One solution to toggle the ToolTip would be to:
Dim tipText as string = ToolTip1.GetToolTip(Button1)
If tipText.Length = 0 Then
' The ToolTip text is not set, add text
ToolTip1.SetToolTip(Button1, desiredText)
Else
' The ToolTip text is set, remove it
ToolTip1.SetToolTip(Button1, "")
End If