The way to read an analog pin and send the result to a virtual pin is to use BlynkTimer to call a function which reads the pin using an analogRead(pin) command and sends the result to Blynk using a Blynk.virtualWrute(vPin) command.
If you want to change the read frequency then you’d need to use a widget (probably a slider or numeric input) to change the timer.
The BLYNK_WRITE() command associated with that widget would need to re-define the timer for these changes to take effect, this would involve deleting and re-creating the timer, and to do that you’d need to know the timer index number.
More information (but not a specific example for your use-case) here…
Pete.