Heya,
I'm a bit confused here. If you take a look at the VGearDoorR3, you'll see it uses the Vault 108 gear door mesh. In the model then, it's set to override the door number with the number 3 from the "Vault11RuinedNumbers" texture set. This texture set contains different vault numbers, as seen here:

http://img339.imageshack.us/img339/610/textureset.png

My question is how does the GECK know to choose the number 3 from the texture set? How can you make the mesh choose another number from the set?

It's not done in the GECK, it's done in the Model itself. What you are seeing is called, "UV Mapping", which is how a 3D model applies a 2D texture over it's surface. The UV map basically says, "Apply this part of a texture to that part of the mesh". It's not Simple, but Echonite provides some excellent tutorials on YouTube that can teach you how to UV Map in Blender (one of the 3D modelling programs we use, the others are 3DSMax and Maya for the most part).

When I didn't know how to UV Map myself, I would modify textures like that to fit my needs by changing the number "3" (for example) to whatever I needed, and then using the same model/texture set to get the results.

I don't have Echonite's YouTube links, but if you search YouTube it shouldn't be hard to find.

Luck,

Miax

never mind.