My problem is actually a bit more complicated than just how to use AES in VB.NET, since what I'm really trying to do is use AES in VB.NET from within a Java application across JACOB. But for now, what I need to focus on is the AES implementation itself.

Here's my encryption code

Public Function EncryptAES(ByVal toEncrypt As String, ByVal key As String) As Byte()
    Dim keyArray = Convert.FromBase64String(key)
    Dim toEncryptArray = Encoding.Unicode.GetBytes(toEncrypt)

    Dim aes = New AesCryptoServiceProvider
    aes.Key = keyArray
    aes.Mode = CipherMode.ECB
    aes.Padding = PaddingMode.ISO10126
    Dim encryptor = aes.CreateEncryptor()

    Dim encrypted = encryptor.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length)
    aes.Clear()

    Return encrypted
End Function

Once back in the Java code, I turn the byte array into a hexadecimal String.

Now, to reverse the process, here's my decryption code

Public Function DecryptAES(ByVal toDecrypt As String, ByVal key As String) As Byte()
    Dim keyArray = Convert.FromBase64String(key)
    Dim toDecryptArray = Convert.FromBase64String(toDecrypt)

    Dim aes = New AesCryptoServiceProvider
    aes.Key = keyArray
    aes.Mode = CipherMode.ECB
    aes.Padding = PaddingMode.ISO10126
    Dim decryptor = aes.CreateDecryptor()

    Dim decrypted = decryptor.TransformFinalBlock(toDecryptArray, 0, toDecryptArray.Length)
    aes.Clear()
    Return decrypted
End Function

When I run the decryption code, I get the following error message

Padding is invalid and cannot be removed.