Da String a MemoryStream

Dim _string As String = "Stringa da convertire"
Dim _bytes As Byte() = System.Text.Encoding.UTF8.GetBytes(_string)
Dim _memoryStream As New System.IO.MemoryStream(_bytes)

Da MemoryStream a String

Dim _bytes As Byte() = _memoryStream.ToArray
Dim _string As String = System.Text.Encoding.UTF8.GetString(_bytes)