I'm investing my house down payment money (planning to use in the next few years) in VBLTX, but not because I think the bond market is currently underpriced, just because I want to keep it somewhere less volatile than VTSAX but still have some room for growth. Right now it's looking like a terrible idea because, as you say, the bond market is down, but I haven't lost too much money and hopefully there will be some upside before I decide to buy.
I didn't actually change my asset allocation much, just moved more of my taxable brokerage account to VBLTX and more of my non-taxable to stocks. If I do end up using the brokerage account money for a down payment I'll rebalance to my original AA.