It's probably worth spending some time reviewing the options. Here is just one article among many you can find with a search on good core bond ETFs or funds : Best ETFs: Medium-Term Bond Funds - Forbes
ETFs tend to be lower in expense. Depending on where you own them, trading may be commision free as well. AGG is a bond index that's often used as the benchmark for diversified intermediate bond funds. It's also low-cost.
The core intermediate bond funds I own are in the 4 or 5 year duration rather than the 6 years of the Vanguard ETFs, but that probably doesn't matter that much for the long run - i.e. for a long term holding period. If you care about having some money available in 1, 2, 3, 4, 5 years, etc. you can accommodate this with a mix of cash, short-term and then intermediate term bonds.
I have a large position in DODIX - that's an example of a core intermediate fund. It's not an index, and it's not as inexpensive as the Vanguard offerings or ETFs. MWTRX is another core bond fund that generally gets good marks. These are just examples.