What risk are you trying to diversify against? When you list Vanguard, American, T Rowe Price...etc, it sounds like you're considering yourself diversified based on that alone. You're not investing "in Vanguard", you're investing in the underlying securities.
You can have 100% of your money "in Vanguard", and be completely diversified across tens of thousands of stocks/bond markets all over the world. On the other hand, you can split up your holdings amongst 10 different companies, but if they are all held in "USA Tech Stock Funds", I wouldn't consider that very diverse at all.