Encoding multiple fields with same AES key and vector? I'd like some advice on cryptography best practices. I need to store on my web app the users' credentials (login &amp; password) for 3rd-party sites.

I'm using AES-256 with random key and initialization vector (provided by the Net 4.5 framework, if that's relevant), and store the encrypted data in completely different system than the key and vector.
Along with the encrypted data record there's the id of the key/vector record in the other system. My question is, is it OK to use the same key and vector to encrypt the login and the password, which are encrypted and stored separately? Or that's a vulnerability and I absolutely need specific key/vector for each?
Speaking in broad strokes, reuse of the key is fine - reuse of the IV: not fine.  From wikipedia: "Properties of an IV depend on the cryptographic scheme used. A basic requirement is uniqueness, which means that no IV may be reused under the same key".  You also need to decide on a mode of operation, as different modes will dictate different requirements for the IV/nonce.