'nell'evento load della form:
dim i as long = 0
dim sFormNames() as string '<-riempi coi nomi delle form che vuoi visualizzare

Do While i < sFormNames.count
                MainTab.TabPages.Add("TAB" & i)
                MainTab.TabPages(i).Height = 999
                MainTab.TabPages(i).Width = 1666

                frmX = CreateForm(sFormNames(i))

                frmX.TopLevel = False
                frmX.Visible = True
                frmX.Top = 0
                frmX.Left = 0

                MainTab.TabPages(i).Controls.Add(frmX)
                MainTab.TabPages(i).AutoScroll = True


                i += 1
            Loop

'e poi ci sono (io li ho messi in un modulo) a parte

Imports System
Imports System.Windows.Forms
Imports System.Reflection

Public Function CreateObjectInstance(ByVal objectName As String) As Object ' Creates and returns an instance of any object in the assembly by its type name.  
        Dim obj As Object
        Try

            If objectName.LastIndexOf(".") = -1 Then 'Appends the root namespace if not specified.

                objectName = [Assembly].GetEntryAssembly.GetName.Name & "." & objectName

            End If
            obj = [Assembly].GetEntryAssembly.CreateInstance(objectName)

        Catch ex As Exception

            obj = Nothing

        End Try

        Return obj
    End Function

    Public Function CreateForm(ByVal formName As String) As Form ' Return the instance of the form by specifying its name. 

        Return DirectCast(CreateObjectInstance(formName), Form)

    End Function