I don't have any accounts at Vanguard but have purchased treasuries at Fidelity, TD Ameritrade and Charles Schwab over the last 18 months (heading into retirement).

Since I don't use Vanguard I don't know how things are listed.

I do know if you search for treasuries the various brokerages will list them and show a yield to maturity. They also show the minimum amount required to be purchased. This number is often quite large (easily 6 figures) to get the max yield. If you put in an order to buy less then the yield will end up being less since the price of the bond is more.

At Fidelity after you search for bonds and sort them whatever way you want (maturity maturity, yield, etc.) there is a button on the far right that says something like "order book" and when I click on that I see different ask prices depending on the quantity of bonds you are purchasing. Going down to a minimum of 1 (which is $1,000).

Maybe the original quote was for a minimum that was higher than the single bond you bought?

Usually you should see this info when you preview your order before you finally commit to the order.

I will point out some brokerages are more lenient than others in terms of when money is available to use to purchase another bond after reaching maturity date. I don't know the details but in some cases, I think Fidelity, will let you immediately use the money to buy another bond on the maturity date while other brokerages will require a day or two. I've also heard if you have an account set up for margin use, you can also immediately buy the next bond. Personally I never get into those details.

I gave up on Vanguard a long time ago. I had some terrible customer experiences when I was listed as a beneficiary on my father's accounts. So I immediately moved everything to Fidelity.