Vigenere (xor version) cipher

[Ivenesco]

Who can improve it? On 1 core 2,8 Ghz it works with speed above 370 MB/s.
Is there any hints, to code it better?

 Function Vigenere(ByRef input() As Byte, ByRef key() As Byte) As Byte()

        Dim intKey As Integer = 0
        Dim x As Integer = input.Length - 1
        Dim x2 As Integer = key.Length
        For i As Integer = 0 To x
            If intKey = x2 Then
                intKey = 0
            End If
            input(i) = input(i) Xor key(intKey)
            intKey += 1
        Next

    End Function

My improvements to first version (much slower than this):
-'ByRef' instead of 'ByVal'
-for i as integer...(and if ... then) ended with variable (x or x2) instead of length calculating each time
-'if intKey = 0' statement to choose key from array is much faster than:

input(i) = input(i) Xor key(i mod key.Length)