BND has a surprising amount of "BBB" (or "Baa") bonds according to Vanguard's portfolio page:
https://investor.vanguard.com/etf/profile/portfolio/bndWow do you have interesting timing - the Fed has literally just lowered rates hours ago. I went to check on current rates, and found they just dropped. Anyways, here's the key line from their statement:
"the Committee decided to lower the target range for the federal funds rate to 0 to 1/4 percent"
https://www.federalreserve.gov/newsevents/pressreleases/monetary20200315a.htmSo, we're now at 0% rates for the Fed, which should translate to a drop in treasury and bond yields. But Vanguard Total Bond Market had a 1.9% SEC yield before the drop, so don't expect that to go all the way to zero.
If all bonds went to 0%, nobody would need 20 year bonds when they can get the same rate on a 1 year bond. So because of the time risk, longer term bonds pay more. And then companies need money, and issue bonds. If they pay the same as the U.S. government, nobody will prefer a company's bonds - so they pay more.
Vanguard Total Bond Market holds a range of bonds, with 2/3rds being government bonds, and roughly 1/3rd being the lower half of investment grade bonds. I wouldn't expect it to hit zero.