http://www.mkv25.net/dfma/movie-1385-clockmechanismtest2 provides an example of how you can delay mechanical timing mechanisms.
Regarding the 'breaking into the vault' problem, you could just put all of the anvils and picks inside the vault before handing over the fort (and um... prevent traders getting into the fort so they don't bring new ones). You would need to put the timing mechanism inside the vault obviously, otherwise it could just be messed with. It wouldn't make the fort very fun though.
You could also try this:
* Put vault on the bottom of map.
* Access vault via floodgates only.
* Have a convenient way to flood/empty vault surroundings with magma to allow access. This is for setup and access purposes.
* Construct a horribly complicated timing mechanism inside the vault. You would need to include an infinite waterwheel power generator to power it.
* Surround vault on *all* sides (including the roof!) with pressure plates which will activate if the magma ever drops below 7/7 - activation causes destruction of vault contents *unless* the timing mechanism has reached its expiry time. Use an atom smasher to destroy the vault contents.
Note: Pressure plates set to trigger on magma still trigger when encased in obsidian (it considers the magma to go from 7/7 -> 0/7 when obsidian forms).
I think that would make the vault almost impenetrable (at least if someone wanted to get the items). It would require a ton of bauxite though. The only potential problem I can see is if someone starts dropping bits of terrain on the vault (thus de-constructing it).
You might be able to get around that problem by putting the vault on the very top of the map, though that suffers from other problems (like people creating a lake underneath the vault and then deconstructing whatever is supporting the vault).
Making an impenetrable vault sounds like a fun challenge
.