So you need help solving those cryptograms. Well, this should
get you started. It will replace the most commonly occuring letter
with an "e". Just paste this code to a form with 2 text boxes
(Text1,Text2) and a command button (Command1).

Code:
Option Explicit
Dim sEncoded As String

Private Sub Form_Load()
  Text1.Word
  'make sEncoded any string you want to decode
  sEncoded = "fzdQlkzqfLLWzEOQzFLzFlQzqgorZtyqZZitQfziqzQitzytZITOZIYTiZtyZSoto"
End Sub

Private Sub Command1_Click()
  Dim i, iNewStart, iCount, iHigh As Integer
  Dim sCompare, sMostCommon, sDecoded, sReplace As String
  Dim aLetterCount(26) as Integer 'alphebet
  Text1 = sEncoded
  
  'find most common letter in encoded text
  For i = 65 To 91 'ascii #s for letters
    sCompare = Chr(i) '(65)=A Chr(97)="a"
    iCount = 0
    iNewStart = InStr(1, sEncoded, sCompare, vbTextCompare)
    Do While iNewStart <> 0
      iCount = iCount + 1
      iNewStart = InStr(iNewStart + 1, sEncoded, sCompare, vbTextCompare)
    Loop
    'make record of # of time a letter occurs
    'aLetterCount(0) is the # of times "A" or "a" occurs
    'aLetterCount(25) is the # of times "Z" or "z" occurs
    aLetterCount(i - 65) = iCount
  Next i 'check next letter
  
  'replace the most common letter with "e"
  For i = 0 To 25 'letters in alphebet
    If aLetterCount(i) > iHigh Then iHigh = aLetterCount(i)
  Next i
  For i = 0 To 25
    If aLetterCount(i) = iHigh Then
      sMostCommon = Chr(65 + i)
      sReplace = "E"
      'Replace(expression, find, replacewith[, start[, count[, compare]]])
      sDecoded = Replace(sEncoded, sMostCommon, sReplace, 1, -1, vbTextCompare)
      Text2 = sDecoded
      MsgBox "The most common letter is: " & sMostCommon & vbCr & "It was replaced with: " & sReplace
      Exit For
    End If
  Next i
End Sub
Note that the # of occurances of each letter are stored in an array
so you can check the 2nd most common occurance, the 3rd, and so on.
Have Fun.