FWIW, here's how M* defines the "yield" it uses:
Yield, expressed as a percentage, represents a fundís income return on capital investment for the past 12 months. This figure refers only to interest distributions from fixed-income securities, dividends from stocks, and realized gains from currency transactions. Monies generated from the sale of securities or from options and futures transactions are considered capital gains, not income. Return of capital is also not considered income NMF--or No Meaningful Figure--appears in this space for those funds that do not properly label their distributions. We list N/A if a fund is less than one year old, in which case we cannot calculate yield.
Morningstar computes yield by dividing the sum of the fundís income distributions for the past 12 months by the previous monthís NAV (adjusted upward for any capital gains distributed over the same time period).
and Here's how Vanguard defines the yield it posts on its website:
A ó THIRTY DAY ADVERTISED YIELD NET OF EXPENSES CALCULATED DAILY
For comparing investing in either fund as of now, I think Vanguard's yields represent the best comparison, as they are roughly the standardized yield
that an investor can receive in the future. So, Vanguard's bond fund yields are taking the interest for the last 30 days and annualizing them, which would be what you'd expect to received in income over say the next year.