Is there any function to set time to scan?

Yes there is.
"Task execution settings" can set up a scannning start date/time and scanning intervals on the Vex interface.

Not only the specific date and time, it also can set up regular schedules, which is useful for conducting nighttime scanning and repeated scanning.

<Procedure>
1. Log in to Vex from the user page.
2. Select a project to be scanned.
3. Click the Web "Scan" tab > "Task execution settings" button.
4. Set the initial date/time to start and scanning interval arbitrarily.
*The scanning plan must be applied in advance.

As a scanning plan is completed, the scan automatically stops.
There is not a function to set stop scanning time on the Vex interface. However operating Vex on the command line with VEX-CLI tool can set stop time. For the details of the commands, refer to the "Vex-CLI Guide" included in the Vex installer.