To set your Voucher Connect store to require CVC verification for all payments on your store, you'll need to log into your Stripe dashboard.  Once you get in to your dashboard, over on the right sidebar you'll see a button labeled Radar.

If you click on that button, it will open a sub-menu where you can find another button called Rules. Click on that to open the rules panel. 

In the middle of the rules panel, there is a pane with the title: Then, when should a payment be blocked?

The third option in this pane is: Block if CVC verification fails, which is disabled and greyed out by default. To enable it, click on the tripe-dot menu button on that row, and click Enable. 

A dialog box will pop up, asking you to confirm your decision. Click the Enable button to proceed.

Once you've done that, all payments will be blocked if they don't have the correct CVC code entered.  You can close your Stripe dashboard whenever you're ready.