I have checked the classs-vendor-signup.php and it looks like the
add_action( 'woocommerce_register_form', array( $this, 'vendor_option' ) );
is not doing its job and is not including the html-vendor-signup.php into the login form.
The prove is that I can get the checkbox to the login form if I paste manually the include_once('views/html-vendor-signup.php'); into the WCV_Vendor_Signup constructor.
I have fixed the problem by using this plugin https://en-ca.wordpress.org/plugins/woocommerce-simple-registration/ and adding a custom register link to my menu structure.
