For a company that prides itself on low expense ratios, a thank-you card seems oxymoronic when they could accomplish the same effect with e-mail. Or even a simple "Thank you for your business!" screen when you made the original transaction...
I agree with this, but one semi-plausible explanation could be that an email or screen prompt does not have the same effect as a tangible, physically delivered card (at least not to all people). And for an explanation of why it may make sense for Vanguard to make any expenditures of this type in the first place, see this post from their blog:
http://vanguardblog.com/2009/11/25/why-we-advertise/
Advertising brings in more customers (and more money) which helps Vanguard scale their business and lower their expense ratios.
But Vanguard spending your money to thank you for giving Vanguard your money... they already have your money. Is a postal-mail card going to inspire you to boost your savings rate, or were you already sending money their way in auto-deductions?
This philosophy is right up there with the friendly financial advisor who sends you "free" courtside tickets to the basketball game. Different orders of magnitude, but were you going to yank your money out of Vanguard because they failed to properly demonstrate their affection for you?
A more tangible expression of their appreciation would be lowering their expense ratios again!
Emotionally and psychologically, I'd have the same "Awwww..." reaction to a thank-you card. But mathematically and financially... we're suckers.