Quick Summary - Vanguard Brokerage updates rarely provide up to date balances throughout the day, and more importantly, don't provide correct balances (often) after close of business (COB). Balances on the accounts on the Transactions tab don't match Market value on the Investments Portfolio tab or the "Balance today" on the Investments Balances tab.
Additional details - the account balances on the Transactions tab is often more correct than the other two screens discussed above. Still, they often don't reflect the correct final balance for the day after COB. FYI, there is usually about a 20 minute delay with Vanguard on account balance information so I typically don't see the final balance as of COB until about 4:20PM, Eastern time zone. My guess (based on my observations) is Simplify does its final update around 4:00PM, Eastern. Why the balances don't match between screens is unknown and I can't find any discussions explaining why. These issues follow between the web page and the mobile apps (iPad and iPhone).
FYI, I'm running a side by side between Quicken Classic (Mac) and Simplifi. I'm enjoying Simplifi and want to make the move so Simplifi permanently when my Classic license expires in October. I have been a Quicken Windows/Mac user for about 30 years and a Quicken Classic Mac Beta Tester for about 10 years. Quicken Classic updates with Vanguard (with the 20 minute lag) all day and closes the day with the correct balance.