If you two believe in each other, you'll WANT to bounce ideas off of each other. You'll be creating the vision together by doing so. Titles are meaningless as far as setting vision goes IMO, especially at an early stage.
My recommendation is to ask yourself if you truly see yourself working at the company for an extended period (10+ years). Realising that the CEO is just another employee of the business and is probably going to face unbelievable stress in their lives over the next few years, possibly lose their house and/or girlfriend/wife, have the business take over every aspect of their lives while earning less than most of your employees, and still having to wear a smile every day despite living through the apocalypse, be grateful that isn't you.
Work hard, support your CEO where you can, as they grow you should to, build something you can really be proud of in terms of the product and the business structures/processes over the next 3/4 years, pick up your shares, and leave with the respect and admiration of your team.
You'll be in the possession of a large chunk of equity that should be growing quickly + your voting rights, not having to sacrifice everything in your life to continue growing the business, and not having to carry the stress that building a successful business brings.
In terms of your practical day to day responsibilities... Every IT business needs an accountant, lawyer and a technician; as you get a little bigger you are going to need a 'CEO', professional assistant and sales/marketing manager. Those roles are pretty clearly defined. My suggestion is find one that you really enjoy doing and write your own job description, exercise some responsibility, but otherwise take everything you don't enjoy about your job and make it someone else's problem. If that means hiring someone to do that work, do that, if you can't hire someone, then ask your ceo how he expects it to get done... Don't be confrontational about it...but it can be a great place to start the conversation around roles and responsibilities. Remember, you're building a start up to have the opportunity to create a place you actually want to work, that also means having the opportunity and the responsibility to craft a job you want. You're not going to get it all your way, but at least you've exercised a lot more freedom than many get to have.
And remember, regardless of your role or position, be respectful, be humble and be a professional and you will always grow.