The surround.vim plugin allows you to wrap text objects with various surrounding characters (e.g. ( ), { }, " "). If you have a visual selection on 1 2 3 4 5 and type S] you will get:

[1 2 3 4 5]

That works, but if you prefer a more readable version with some extra breathing room, you can make the visual selection and hit S[ which will stick a space on either end:

[ 1 2 3 4 5 ]

Now, if you already have some text wrapped in square braces, like the first example, and you want to convert it to the more spacious second example, you can do a change surround command followed by hitting the open square brace twice (that is, cs[[) which will convert [1 2 3 4 5] to [ 1 2 3 4 5 ].