This is c# syntax, but the classes should all be the same for VB.net. You would need to know the padding scheme (if any) and the cipher mode used on the encryption routines. Its a fair bet that if an IV isn't being used, then its using ECB mode.
Its also important to get the encodings correct when building the byte arrays containing keys and encrypted data. It may be ASCII, Unicode, UTF...
using System.Security.Cryptography;
using System.IO;
byte[] encryptedBytes = new byte[16]; // multiple of 16 (blocksize is 128 bits)
byte[] keyBytes = new byte[16]; // if keysize is 128 bits
Rijndael rijndael = Rijndael.Create();
rijndael.Mode = CipherMode.ECB; // But try other modes
rijndael.Padding = PaddingMode.None; // But try other padding schemes
rijndael.BlockSize = 128;
rijndael.KeySize = 128;
rijndael.Key = keyBytes;
ICryptoTransform cryptoTransform = rijndael.CreateDecryptor();
MemoryStream ms = new MemoryStream();
CryptoStream cs = new CryptoStream(ms, cryptoTransform, CryptoStreamMode.Write);
// Write the data to the stream to perform the decryption
cs.Write(encryptedBytes, 0, encryptedBytes.Length);
// Close the crypto stream to apply any padding.
cs.Close();
// Now get the decrypted data from the MemoryStream.
byte[] decryptedBytes = ms.ToArray();