Вот еще один способ сделать это - с помощью функции API WideCharToMultiByte:
Option Explicit
Private Declare Function WideCharToMultiByte Lib "kernel32.dll" ( _
ByVal CodePage As Long, _
ByVal dwFlags As Long, _
ByVal lpWideCharStr As Long, _
ByVal cchWideChar As Long, _
ByVal lpMultiByteStr As Long, _
ByVal cbMultiByte As Long, _
ByVal lpDefaultChar As Long, _
ByVal lpUsedDefaultChar As Long) As Long
Private Sub getUtf8(ByRef s As String, ByRef b() As Byte)
Const CP_UTF8 As Long = 65001
Dim len_s As Long
Dim ptr_s As Long
Dim size As Long
Erase b
len_s = Len(s)
If len_s = 0 Then _
Err.Raise 30030, , "Len(WideChars) = 0"
ptr_s = StrPtr(s)
size = WideCharToMultiByte(CP_UTF8, 0, ptr_s, len_s, 0, 0, 0, 0)
If size = 0 Then _
Err.Raise 30030, , "WideCharToMultiByte() = 0"
ReDim b(0 To size - 1)
If WideCharToMultiByte(CP_UTF8, 0, ptr_s, len_s, VarPtr(b(0)), size, 0, 0) = 0 Then _
Err.Raise 30030, , "WideCharToMultiByte(" & Format$(size) & ") = 0"
End Sub
Public Sub writeUtf()
Dim file As Integer
Dim s As String
Dim b() As Byte
s = "äöüßµ@€|~{}[]²³\ .." & _
" OMEGA" & ChrW$(937) & ", SIGMA" & ChrW$(931) & _
", alpha" & ChrW$(945) & ", beta" & ChrW$(946) & ", pi" & ChrW$(960) & vbCrLf
file = FreeFile
Open "C:\Temp\TestUtf8.txt" For Binary Access Write Lock Read Write As #file
getUtf8 s, b
Put #file, , b
Close #file
End Sub