Are we talking about VBISX (
https://investor.vanguard.com/mutual-funds/profile/performance/vbisx)?
If you look at the info, as audrey points out, there is risk in owning it as opposed to CDs or other cash-like instruments. This fund is currently holding 65% government securities and 35% corporate bonds.
Look at the performance page - YTD is a
loss of 0.28% and 1-year performance is a
loss of 0.5%, that includes your yield. Additionally, this fund lags its index in performance (which also shows a 1-year loss).
For that performance, as the overview indicates (
https://investor.vanguard.com/mutual-funds/profile/overview/vbisx), the duration is 2.7 years. Additionally, you are paying 0.15% expenses.
For 2.7 years duration, you could have CDs paying ~3.2%, with no fees and with no risk.
Again, total return for the past year of the fund is a loss.
That is why you should bother with CDs.