When I did an exchange like this, I called up Vanguard on the phone and had them do what I wanted. No intermediate money market. As for 1 or 3 transactions on a 1099B, I can't recall because it really is not an issue.
The trick is to tell them "and all accrued dividends", so you are not left with any potential partial monthly bond dividend at any point.
It's all about market timing, uh, I mean rebalancing, uh, I mean opportunistic rebalancing, uh, I mean short-term opportunistic rebalancing due to a short-term change in one's asset allocation.