I never understood Vanguard's recommendation for foreign bonds. Their stated reason is that it provides some diversification benefits.
I agree that is true for diversifying returns, but the primary benefit of bonds is to reduce your risk in the event of a stock market crash (preserve your capital.) Foreign bonds introduce currency risk. During a market downturn, that currency risk impact is unpredictable. Money might flee the dollar, or flee to the dollar, depending upon what the crisis was that triggered the downturn.
During normal markets, the foreign currency risk may also help or hurt your returns. But bonds aren't there for long-term returns. That's what equities are for.
Not understanding why you would ever buy foreign bonds, I stay with US-based bonds. I do have significant foreign equity allocations (which is where I choose to take my currency risk.)