Well...
Try this:
TextBox2.Text = TextBox2.Text & word & vbCrLf & "5" & word
[EDIT]
Or use custom function, like this:
Sub Main
Dim inputstr = "5156785123579"
Dim splitter = "5"
Dim data As List(Of String) = CustomSplit(inputStr, splitter)
For Each s As String In data
Console.WriteLine(s)
Next
End Sub
Function CustomSplit(StringToSplit As String, SplitBy As String) As List(Of String)
Dim retVal As List(Of String) = New List(Of String)
Dim l As Integer = StringToSplit.Length
Dim currIndex As Integer = 1
Dim prevIndex As Integer = 0
Do
currIndex = StringToSplit.IndexOf(SplitBy, currIndex, StringComparison.InvariantCulture)
If currIndex >-1 Then
retVal.Add(StringToSplit.Substring(prevIndex, currIndex -prevIndex))
previndex = currIndex
currIndex += 1
Else
currIndex = l
retVal.Add(StringToSplit.Substring(prevIndex, currIndex - prevIndex))
End If
Loop While currIndex < l
Return retVal
End Function