If you steal legendary items from a locked vault by loba ult, you can do it solo. You need to use the loba ult twice to steal the helmet and body armor from the vault. (The first ult takes 60 seconds, the second and subsequent ones 120 seconds.) There are three vaults in World's Edge, but one recommended location is near THE DOOM and LAVA CITY, because the other two places are more likely to be visited by many enemies.
Another way to do this is to get a vault key from a cargo bot and open the vault. This method is a bit more difficult because it involves luck and you have to play solo or with your friends, otherwise your allies may take your items.
Edit: Usually you can steal two items with loba ult. But if you steal an item from the vault, ult will break. So you need to use loba ult twice. Also, the probability that the helmet and armor are in the vault at the same time is not 100%, so you need to repeat the trials until the two items appear at the same time.