And now here's the whole list for April:
https://personal.vanguard.com/us/insights/article/expense-ratio-changes-042015About 20 funds saw the ER go down, while a handful (funds no one cares about) went up.
The funds in the list are only those that update their yearly prospectus at the end of April. Other funds update the prospectuses in different months.
And my understanding is that, at least for Vanguard, these ERs are backward-looking numbers. It's not "hey guys, what are we going to charge for this fund going forward?" Instead, it's "What were our expenses? What are our assets? Divide one by the other, and then we can update the ER we publish in our prospectus".
The point being, we were probably
already "paying" 0.07% for VBTLX; the updated prospectus just recognizes that fact.
Also, the whole reason is surely because about a month ago, I moved my Roth IRA holdings from VUSUX to VBTLX (market timing?!) That enormous inflow of assets to VBTLX is obviously why the ER dropped. You're welcome!