We struggled with this issue too.
Ultimately, we set up a brokerage account called "Vanguard Advantage". Quicken then allows you to create a checking account linked to the brokerage account - "Vanguard Advantage-Cash". The linked account is then set up in Quicken like any other account in the cash flow center (e.g. checking). Finally, we set up the sweep money market account as "Vanguard Sweep" in Quicken as a seperate Investment Account. (We had Vanguard assign our linked money market account an account number different from our other taxable investment accounts to keep it cleaner in Quicken.)
Essentially, we have two tranasctions for each payment or deposit. When an item is paid from the checking, Quicken downloads both the sale of the linked money market sweep shares and the payment from the linked checking account. The only thing I need to really deal with is editing the sweep money market sale transaction to transfer the cash proceeds to the checking account ("SoldX"). To do that, in the edit sell box, you will see an option "Record Proceeds?" to this account's cash balance or to another account to be selected. Choose your linked account where the payment was withdrawn. You should end up with $0.00 cash balance in each account when everything has cleared.
It's just the reverse with deposits. When there's a deposit to the checking there will be a corresponding purchase in the sweep. Just edit the bought transaction in the sweep account to have the cash for the purchase taken from the linked cash account ("BoughtX") where the deposit was made.
It's a little more work, but our interest in the PMM fund is 20 times what we were getting in our credit union checking account.
It's definitely worth it.
I hope this helps. If you come up with a more efficient method though, I'd love to hear about it.
