Warning: Undefined array key "vendor"

This is due the config assume the payment gateway has been named sagepay_opayo

public function getSagepayConfiguration(OrderInterface $order = null): array {
    $gateway_configuration = [];

    // Get the payment gateway configuration.
    if ($sagepay_opayo_configuration = \Drupal::config('commerce_payment.commerce_payment_gateway.sagepay_opayo')) {