I've found it to be useful to consolidate at Vanguard. Fewer statements in the mail, and fewer passwords to remember. In addition, you'll qualify for Voyager status at $100K total assets, "Voyager Select" at $500K and "Flagship" at $1M, and consolidating accounts helps people attain these levels more rapidly. These come with some benefits (special account reps and services, etc).
There's virtually no increased risk from putting all your assets with Vanguard (or any other large, solid management company)
Still, I elected to put some money with Fidelity, because Vanguard didn't offer a special kind of account I needed (a solo 401K). So, sometimes it's tough to simplify.