Earlier versions of the CP210x VCP driver purposefully did not allow the PC to enter standby mode when a handle to the CP210x was open. If the PC attempted to enter standby while the handle to a CP210x was open, the following message was displayed:
The VCP driver did this to prevent any errors from occuring during the Standby process (blue screens).
In order to place the PC into Standby when a CP210x was connected, the application had to close the handle to the CP210x. The application did this after any long idle period (please see this article for information on using the built-in Windows Timers).
Newer versions of the driver provide the capability to enter the standby state. You can download the latest drivers here: https://www.silabs.com/products/mcu/Pages/USBtoUARTBridgeVCPDrivers.aspx and run the self extracting executable to install the drivers.