Is it a bad idea to invest in several mutual funds from the same fund family?

I'm thinking of investing in two Vanguard funds (investing $3000 in each), but I don't know if this goes against the laws of diversification. This is about half of my total investment money. I wouldn't put ~50% in Vanguard if the minimums weren't so high.

Why Vanguard? Low expense ratios. That's the only reason.

This is for long term retirement investing.