If a bank vault survived a nuclear bomb, you would not be able to get in them with a lock pick...
Not to mention they would be covered i the rubble of the rest of the bank.
You have not thought this through.
Also how many banks are there in DC?
Now how much of DC is left be seen?
Now how likely is it the building would survive?
It would be very unlikely to be able to get to a Pre-War bank vault unless you have a digger...
Yes banks would be in sight but, be able to get to them?
Also, DC should be alot worse off than it is made in Fallout 3... Be thank full there are buildings even stood up.
Nuclear bomb's pressure waves do not do good for structures with high centers of gravity...
Actually we can only really assume there was one high-yield nuclear weapon, rarely used, if at all, in the Chinese nuclear arsenal, used for destructive purposes alone, since there was only one ground zero, low-yield nuclear weapons and neutron bombs, both commonly used in the Chinese nuclear arsenal, would have mopped up other targets surrounding DC. This would have been more advantageous for an enemy to use especially as technology progressed and the war for resources heated up, why destroy what you will need?
Now, I'm no physicist, but suburban and rural areas being as destroyed as they were only made sense because of the warfare in the region over the 200 some years after the Great War took place, from what happened in Nagasaki and Hiroshima we know how the blast radii work and how they can be intensified based on their yield. There should have been more than one ground zeros, therefore the use of neutron bombs would have been more plausible.
If that were the case for every building, maybe, but it isn't. Not just banks are going to be inaccessible. I would think the bank vaults, on the other hand, would survive nuclear blasts, and not all covered underneath rubble, not just office buildings.