First, the primary reason is shelter and access. Use a FEMA rated shelter door. Avoid residential grade features as much as possible.
More than that will be above and beyond the strength of the concrete block and it would be easier to penetrate the walls to get in. Balance the construction features of the walls to the door because going overboard will be overspending and only direct the attention of the experienced to the weaker materials.
The construction should be evaluated in terms of minutes of resistance - a determined attack by nature or a thief is basically a matter of time. As we learned in the EF5, 250 mph winds will basically throw cars into construction. If it's not reinforced poured concrete anchored to the foundation with it's own structural integrity separate from any roofing it will fail. All the tilt up commercial buildings came down when the roofs blew off.
That's why a door of much more resistance is a waste of time - a wall breach is often easier. Place a gun safe anchored inside that room and use the rest of it to store recovery essentials like water, food, hard use clothing, and extraction tools. Dedicate them to that purpose, don't take things out of there as they will be when you need them.
Be careful how the addition is tied into the existing construction as the demolition of the other parts of the house can't be allowed to drag down the vault room. And consider carefully how flooding my affect things - I live on a hill, yet the basement has had three inches of water in it due to the builder not properly pouring the foundation. When the table rises it penetrates the joint at floor level and pours in. Plan for a 100 year flood event, it's the amount of rainfall in 12 hours that will flood you at your site, your specific drainage is the source of most flooding. I have a creek 25 feet lower than the house 100 feet behind it and that isn't my problem.
