riddle - Help! I'm dying!

You are woken up to hearing your neighbour shouting - "HELP! HELP! I'M DYING!"

You jump to your feet and run out into the street and see a man run away from your neighbours house, but he is too far away for you to catch up with him, and he is wearing a mask. You decide to investigate the house, as you are an ex-policeman. You use your spare keys to go inside the house and see the body on the floor, dead. You also see a bit of paper sticking out of his pocket. You pick it up. It reads:

If you are reading this I am dead, but I believe I know who killed me. The answer is encrypted below.

eUXODGaaSDW8WURaLHaNWvjP2YNVJQWMSEdOWVQMSPdM9GaMJVjA6VQMJdRNWOhi7DcCATbiKRW

Damn it. Your neighbour loved cryptography. Oh well, you may as well have a go, you suppose. Then you notice something else sticking out of his pocket. Another bit of paper:

I've always had an extensive love of doughnut

What relation to him was the person that committed the murder?

NOTE: The reason of the puzzle being tagged "riddle" will be revealed later...

Hint 1:

Only two words on the second bit of paper are important.

Hint 2:

The two important words are "extensive" and "doughnut".

Hint 3:

CodeNewbie has guessed (correctly) that it has something to do with a Vigenere Cipher. However, only one word is the Vigenere Cipher key. The other is something else...

Answer

The murderer is

The neighbour's son.

The message is encrypted using

an Extended Vigenere Square, which includes the characters A-Z, a-z, 0-9, and a space.

The decryption key, as mentioned by the hints, is

doughnut

The decrypted text reads (punctuation added)

Brothers and sisters I have none, but the murderer's father is my father's son.

This means that

the murderer's father is the victim, and thus it is the victim's son or daughter that killed him. Based on the clue about a masked man running from the house, I'm going to go with "son." Hopefully he has only one, or the case could be tied up for a while in court... :)