Yeah, the wooden bumper thing. That, or just make it look like the tracks are supposed to go inside (slightly elevated on a ramp, etc).
A random thought, but why not have the bomb go inside?
Maybe the RED engineer's worked up a "automated delivery system", where the carts filled with "gold" would pull up to the vault, and it's lowered down into the ground on a platform and moved inside the vault that way so they don't have to open the door all the time and leave the contents exposed to theft. (And hey, you can make up a "All deliveries in the rear." sign, too.)
What RED didn't expect was a BLU engineer to find out about this, and devise a plan to mock up a bomb so it looks like a standard "delivery" to the system. A little clever Spy-work, and the system's now all expecting a gold shipment.
RED, on the other hand, discovered the problem at the last second, but couldn't shut down the system since it was tied fully into the vault's security system, so they've sent a team out to keep this "payload" from being delivered by any means necessary.
At the very minimum, having the bomb delivered inside the vault would give a excuse for the door and parts to fly outward, which might be a bit more impressive from a visual point-of-view. Sort of Goldrush/Badwater-ish, where the eruption from the pit upwards sending concrete bits flying everywhere looks more impressive than if the bomb would just blow up and collapse the "hole" inwards onto itself. It does look a little odd for some of the door bits to be flying in the wrong direction from the blast, unless they're bouncing off the door from the impact.