What do the secret vault symbols look like?

Solution 1:

It is the same symbol that was all over Borderlands 1, and is still the 'saving' icon in Borderlands 2.

As you explorer them, you will find some small (like this screenshot), some large, some half obscured by objects, some on floors, some on walls, etc.

(screenshot from the video guide for the 5 symbols in Sanctuary)

Solution 2:

This answer deals with finding an easy example in-game, and as such contains spoilers:

Go to the very first area (Claptrap's 'Home') where the beast pulls Claptrap's eye out and you're asked to search the locker or something for a gun. It's the first gun you get in-game, in the area with tons of dead Claptraps.

On the right side of that area is a wall you can open. A bunch of dead claptraps will burst out when you open it. The borderlands symbol is there to "discover" on the back wall.

I have yet to find any others though.