Let's look at it in terms of overall expense ratio.
Year 1: $10/$3000 = 0.33% + 0.20% = 0.53%
Year 2: $10/$6000 = 0.17% + 0.20% = 0.37%
Year 3: $10/$9000 = 0.11% + 0.20% = 0.31%
Year 4: over $10K = 0.20%
Obviously, 0.53% is higher than 0.20% but the question is who ELSE can you go to w/ super low fees and no minimums. I myself did a lot of looking in the past for index funds with low minimums and only company I can think of is T. Rowe Price. They have about 5 different index funds -- expense ratios look to be about 0.35%. Which means after the 3rd year, Vanguard funds will be charging less.
Did you find the T. Rowe site to be hard to navigate. That's why I went looking at V. in the first place, I couldn't find what I was looking for at T. Rowe.
My alt. is T Rowe and like Mossy SF points out, V beats them by overall expense ratio after a few years. The advantage to T. Rowe is the $50 asset builder. V. has $100 minimum addt'l investment. I think it's reasonable too. I just like to check and see if there are things that I am missing in my research.