Could you provide some additional information about how you have written the scheduled job? Normally accessing ImageVault thru the API will not cause the site to stop working, but it is hard to give any advice on how to address this without more information.
If you also could provide the stack trace from the error we might be able to give you a hint to the cause.
I have looked at the code provided, but fail to see the big picture. Could you please provide a more complete example? Email my directly if you don't wan't put it on the forum. From the description it sounds like some kind of locking issue but it is hard to draw any conclusions with out being able to see how everything is tied togheter.
It might also be possible to get more information by turning on logging. If it is possible to narrow down the scope of where the looking problem might be located it will help a lot.
Also if it is possible to give me a stack trace regarding the problem you had, that would be most helpfull.