I had problems with VB not consistently centering my form in its parent too. So I created the follow bit and stuck it in a module for reusage.
Sub CenterInParent(ByVal parent As Form, ByVal child As Form)
Dim frmA As Form = parent
Dim frmB As Form = child
Dim x As Integer = (parent.Width / 2) - (child.Width / 2)
Dim y As Integer = (parent.Height / 2.2) - (child.Height / 2)
child.Location = New Point(x, y)
End Sub