Stupid Problem, with Replace

[tHa-gRiM]

Im taking a VB class and cant figure out how to get it to replace more than one character this is my code.

Option Explicit On 
Option Strict On
 
Public Class JacobsonForm
    Inherits System.Windows.Forms.Form
 
    Private Sub uiExitButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles uiExitButton.Click
        'ends the application
        Me.Close()
    End Sub
 
    Private Sub uiDisplayButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles uiDisplayButton.Click
        Dim oldpass As String = Me.uiOldPassTextBox.Text
        Dim oldpassnumber As Double
        Dim newpass As String
        Dim i As Integer = 0
 
 
        oldpassnumber = oldpass.Length
        If oldpassnumber >= 5 AndAlso oldpassnumber < 8 Then
 
            Do Until i = oldpassnumber
                newpass = oldpass.Replace("A", "X")
                newpass = oldpass.Replace("E", "X")
                newpass = oldpass.Replace("I", "X")
                newpass = oldpass.Replace("O", "X")
                newpass = oldpass.Replace("U", "X")
                i = i + 1
            Loop
        End If
 
        Me.uiNewPassLabel.Text = newpass
 
 
 
 
    End Sub
End Class

what am i missing i tried to get it to loop, but it only does the U, how can i get to do them all?