You can simply change response charset and content-type with next commands:

  Response.Charset = "windows-1250"
  Response.ContentType = "text/plain" 

    But if you try to write a string data using Response.Write method, the result will confuse client side - the response data will be encoded as utf-8, but response header will have charset=windows-1250. So you have to convert the unicode data to data with windows-1250 code page. I created short function to convert such data from Unicode (VB String) to another code page (for example windows-1250). I created a simple conversion function using System.Text.Encoding:

Function EncodeString(ByRef SourceData As String, ByRef CharSet As String) As Byte()
  'get a byte pointer To the source data
  Dim bSourceData As Byte() = System.Text.Encoding.Unicode.GetBytes(SourceData)

  'get destination encoding 
  Dim OutEncoding As System.Text.Encoding = System.Text.Encoding.GetEncoding(CharSet)

  'Encode the data To destination code page/charset
  Return System.Text.Encoding.Convert(System.Text.Encoding.Unicode, OutEncoding, bSourceData)
End Function

Then you can simply write:

Dim Data As string

' Fill the string variable with some data
Data = "Some string with any chars suitable For windows-1250 charset"

'Set content-type
Response.ContentType = "text/plain"
'Set out charset
Response.Charset = "windows-1250"
'Write string converted To the charset.
Response.BinaryWrite EncodeString(Data, "windows-1250")

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