I have to correct this a bit. If you really want better safety, I think you want to avoid mortgage backed securities and corporate bonds altogether:

http://www.fundadvice.com/articles/retir...

Basically stick to short/intermediate tax free bonds, federal bonds (i don't think vanguard has intermediate ones), and I'd also think about an intermediate bond index in addition to short federal, since it yields more and is much more diversified than corporate bonds.