That’s a great point. This time was a bit different due to very technical reasons. Some Hedge Funds were unwinding risk parity trades which caused turmoil in the Treasury Market (read a good article about this here: https://www.ft.com/content/efa431c8-6471-11ea-b3f3-fe4680ea68b5#comments-anchor)

Generally, the more ‘pure’ your ETF (in terms of allocation to Treasuries) the more it will protect you. TLT recovered in a few days due to these technical aspects but usually trades up with any negative newsflow these days

VBTLX is only 60% Treasuries so will take more time to recover, generally speaking. But it does provide you with yield. That’s the trade off you’re making.

That said, if you’re re-balancing your portfolio e.g. quarterly a few days of volatility won’t have a major impact and you will still reallocate to cheaper Equities.