Public Randomize_thread As New Randomize_thread_class
...
Randomize_thread.StartBackgroundTask()
...

 
Imports PlayLists.Form1
 
...
 
    Public WithEvents Randomize_thread As New BackgroundWorker
 
    Public Sub InvokeControl(Of T As Control)(ByVal Control As T, ByVal Action As Action(Of T))
        If Control.InvokeRequired Then
            Control.Invoke(New Action(Of T, Action(Of T))(AddressOf InvokeControl), New Object() {Control, Action})
        Else
            Action(Control)
        End If
    End Sub
 
...
 
    Public Sub Randomize_thread_DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs) Handles Randomize_thread.DoWork
        Dim lines = File.ReadAllLines(PlayLists.Form1.Temp_file)
        Dim nLines = lines.Count
        Dim randomNumbers = RandomList(nLines)
        Dim tmpFile = Path.GetTempFileName()
 
        InvokeControl(PlayLists.Form1.progresslabel, Sub(x) x.Visible = True)
        InvokeControl(PlayLists.Form1.ProgressBar1, Sub(x) x.Maximum += nLines)
        Using sw = New StreamWriter(tmpFile, False, Encoding.UTF8)
            For i = 0 To nLines - 1
                sw.WriteLine(lines(randomNumbers(i)))
                InvokeControl(PlayLists.Form1.ProgressBar1, Sub(x) x.Value = i)
                InvokeControl(PlayLists.Form1.progresslabel, Sub(x) x.Text = "Randomizing " + i.ToString() + " of " + (nLines - 1).ToString() + " files.")
                ' InvokeControl(PlayLists.Form1.progresslabel, Sub(x) x.Refresh())
                ' InvokeControl(PlayLists.Form1.ProgressBar1, Sub(x) x.Refresh())
            Next
        End Using
        InvokeControl(PlayLists.Form1.progresslabel, Sub(x) x.Text = "All files launched")
    End Sub

    Sub MITHREAD()
       msgbox("hello world")
    End Sub
 
Dim T_first_instance As System.Threading.Thread = New Threading.Thread(AddressOf MITHREAD)
T_first_instance.Start()
 
' esperamos a que el thread finalice y entonces lo volvemos a llamar cuando necesitemos volver a usarlo, pero creandolo con otro nombre de variable:
Dim T_second_instance As System.Threading.Thread = New Threading.Thread(AddressOf MITHREAD)
T_second_instance.Start()
'
' EL ADREESOF ES EL MISMO PARA LAS DOS INSTANCIAS YA QUE LA INTENCIÓN ES PODER USAR EL MISMO THREAD UNA Y OTRA VEZ.

 
sub button1_clicked... 
       Dim RandomClass As New Random()
        Dim RandomNumber As Integer = RandomClass.Next()
        Dim T_(RandomNumber) As System.Threading.Thread = New Threading.Thread(AddressOf MITHREAD)
        T_(RandomNumber).Start()
end sub