VB.NET - VBScript - Insert text into the middle of a string

I am reading a text file line by line checking for specific criteria..

Do While objFile.AtEndOfStream <> True
  strCurrentLine = objFile.ReadLine
if InStr(226, strCurrentLine, "7141570", vbTextCompare) = 226 Then

>Checking Column 226 for this text "7141570", when this returns TRUE I want to insert "99" elsewhere on that line (lets say Column 250).  How do I do this?

InStr([start,]string1,string2[,compare])

start   -->	Optional. Specifies the starting position for each search. The search begins at the first character position by default. This parameter is required if compare is specified
string1 -->	Required. The string to be searched
string2 -->	Required. The string expression to search for
compare -->	Optional. Specifies the string comparison to use. Default is 0

Can have one of the following values:

     0 = vbBinaryCompare - Perform a binary comparison
     1 = vbTextCompare - Perform a textual comparison

as follows

strCurrentLine = objFile.ReadLine
If Not InStr(226, strCurrentLine, "7141570", 1) = 0 Then
  'here u replace the "7141570" string with "99" as u wnat
  strCurrentLine = strCurrentLine.Replace('7141570','99')
End If

Public Function InsertString( _
      ByVal vDest As Variant _
    , ByVal vSource As Variant _
    , Optional ByVal vInsertPosition As Variant _
    ) As Variant

Insert one string into the middle of another string and return the results.
vSource is inserted into vDest before character position vInsertPosition.

    InsertString("123456", "NEW", 3) = "12NEW3456"

    http://www.entisoft.com/estools/StringManipulations_OverlayString.HTML Function
    http://www.entisoft.com/estools/StringManipulations_DeleteString.HTML Function
    http://www.entisoft.com/estools/StringManipulations_StrIns.HTML Subroutine

vDest: The string into which the other string is to be inserted. vDest defaults to an empty string if it is Null or cannot be fixed up to a String.

vSource: The string which is to be inserted into the other string. vSource defaults to an empty string if it is Null or cannot be fixed up to a String.