A secessionist vote is not allowed by Spanish constitution and that is why it was declared illegal.
If you want to vote then first try to change the Spanish Constitution to allow a secessionist poll. If you manage it then try to change the European Constitution that also prevents secessions within member states (1).
Regarding allowing Catalonia in the EU, how can you dream that in a hypothetical secession (that will never happen), after stealing part of the Spanish country, that country is going to have any good relations with you?
If a burglar comes to your home and steals your property, are you going to be friends with him??? Come on, get real! You will want him to be behind bars and to get back your property.
So if secession is currently illegal under Spanish Constitution and European Constitution was is the point of making an illegal, irrelevant and useless vote? Obviously none.
As I said, change both those constitutions first and then we can start talking about all Spaniards deciding if they break Spain apart.
From your comments I also see that you are a xenophobe, of course... what I'm thinking off... secessionism is xenophobism and fascism... hahaha
(1) http://www.venice.coe.int/webforms/documents/default.aspx?pdffile=CDL-INF(2000)002-e
Ummm... a good question...
It was a branch of the Illuminati defecting from the main illumination boss, the Monarchy of England, so they could build their own country. Remember that there was a war as result... Check Washington DC, the Seal of States, the US$ note, etc... to find all the illuminating symbols.
Then those illuminated took on the task of killing practically all of the aborigines from America.
Now that you mention the United States of America... what do you think of the Secession War or American Civil War? You know, the Confederates trying to secede from the United States of America...