You can turn a Form class back to a child control by setting its TopLevel property to False. It becomes essentially a UserControl with some unused overhead. Make it look similar to this:
Public Class Form1
Public Sub New()
InitializeComponent()
Dim frm As New Form2
frm.TopLevel = False
frm.FormBorderStyle = Windows.Forms.FormBorderStyle.None
frm.Visible = True
frm.Dock = DockStyle.Fill
TabPage1.Controls.Add(frm)
End Sub
End Class