Need help with the logic of break a vigenere cipher knowing a partial plaintext

[Elpes Cado]

for example, the encrypt code is "ARJHNRIUCGQHPFGQFHTXPDEHTLNOC", and the part that I ​know of the plaintext is "DERUNA". The key is unknow.

The key is period of 2, so the key are 2 letters. Each letter of the code would specify in turn the magnitude of the cyclical shift.

How i can find the key with c++? And how I can break the cipher?

I have to create a program in c++ that automatically decrypt the message.
(I already know the key, I need you to help me with the logic in c++, not to tell me the key)

Thanks in advance.

[seeplus]

Forgetting code for the moment. How would you solve this using pen & paper? when you know that, then devise an algorithm and from then produce a program design. Then once you have the program design, then code the program from this. The last thing you do is to start to code.