How secure is a vault below a conventional slab for use as a shelter against tornados? I live in South Mississippi and own a house on a slab...

I live in South Mississippi and own a house on a slab with a underground store underneath it, therefore the ceiling of the underground room is concrete and it is a very nontoxic structure. I am contemplating moving to Arkansas and am looking at a house with a conventional crawl space type foundation. It also have a basement beneath it but, I am concerned how safe this type of subterranean vault would be in a tornado. Does anyone enjoy any experience or research information that could help security my fears or would it be best to try and find another house.
Answers:    I would think that any one will give you plenty of protection.. if a tornado his any house, it will destroy the house but what is within the basement will be safe and sound.. regardles of whether you have a cement roof or another manner.. if a tornado blows the house down.. and will be gone.. if you are below ground.. you will still be in one piece..the house will budge, but you will stay..
You should be fine in a vault. But there are more and more houses surrounded by the midwest that have tornado shelters built beneath ground, sometimes accessible thru the garage. The shelters are also available as an add on for any house if you hold room and $. You might look for something like that. A house on a slab near a basement below that seems pretty unusual surrounded by these parts.