Override PlaceOrder and change Grand Total

Question

I am modifying the placeOrder function of the QuoteManagement class, based on some information I found through searches, I was able to change the method so that an order is generated for each seller, but the values are kept the same, ie I get the value total of all items and not just the value of the already separated items. Can someone tell me what place or what should I do so that the total amount is separated according to the items of each order?

PlaceOrder

public function placeOrder($cartId, PaymentInterface $paymentMethod = null)
{
    $quote = $this->quoteRepository->getActive($cartId);

    $paymentMethodString = $quote->getPayment()->getMethod(); // edit 19.10.17

    // get data from addresses and remove ids
    $billingAddress = $quote->getBillingAddress()->getData();
    $shippingAddress = $quote->getShippingAddress()->getData();
    unset($billingAddress['id']);
    unset($billingAddress['quote_id']);
    unset($shippingAddress['id']);
    unset($shippingAddress['quote_id']);

    $itemsPerVendor = [];
    foreach($quote->getAllItems() as $item) {
        // I don't know how do you keep track of your product vendor, use this as reference only
        $product_id = $item->getProduct()->getId();
        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $product = $objectManager->create('Magento\Catalog\Model\Product')->load($product_id);

        $itemsPerVendor[$product->getSellerId()][] = $item;
    }

    foreach($itemsPerVendor as $vendor => $items) {
        // init Quote Split
        $quoteSplit = $this->quoteFactory->create();
        $quoteSplit->setStoreId($quote->getStoreId());
        $quoteSplit->setCustomer($quote->getCustomer());
        $quoteSplit->setCustomerIsGuest($quote->getCustomerIsGuest());
        if ($quote->getCheckoutMethod() === self::METHOD_GUEST) {
            $quoteSplit->setCustomerEmail($quote->getBillingAddress()->getEmail());
            $quoteSplit->setCustomerGroupId(\Magento\Customer\Api\Data\GroupInterface::NOT_LOGGED_IN_ID);
        }

        // save quoteSplit in order to have a quote id for item
        $this->quoteRepository->save($quoteSplit);

        $subtotal = 0;  

        foreach ($items as $item) {
            // add item
            $item->setId(null); // init item id for force to be added to quoteSplit collection
            $quoteSplit->addItem($item);           
        }

        // set addresses
        $quoteSplit->getBillingAddress()->setData($billingAddress);
        $quoteSplit->getShippingAddress()->setData($shippingAddress);


    // recollect totals into the quote
    $quoteSplit->setTotalsCollectedFlag(false)->collectTotals();

        // set payment method // edit 19.10.17
        $quoteSplit->getPayment()->setMethod($paymentMethodString);
        if ($paymentMethod) {
            $quoteSplit->getPayment()->setQuote($quoteSplit);
            $data = $paymentMethod->getData();
            $quoteSplit->getPayment()->importData($data);
        }

        $this->quoteRepository->save($quoteSplit);
        // dispatch this event as Magento standard once per each quote split
        $this->eventManager->dispatch('checkout_submit_before', ['quote' => $quoteSplit]);

        $order = $this->submit($quoteSplit);
        $orders[] = $order;


        if (null == $order) {
            throw new LocalizedException(
                __('An error occurred on the server. Please try to place the order again.')
            );
        }

    // disable origin quote
    $quote->setIsActive(false);
    $this->quoteRepository->save($quote); // edit 19.10.17

    $this->checkoutSession->setLastQuoteId($quoteSplit->getId());
    $this->checkoutSession->setLastSuccessQuoteId($quoteSplit->getId());
    $this->checkoutSession->setLastOrderId($order->getId());
    $this->checkoutSession->setLastRealOrderId($order->getIncrementId());
    $this->checkoutSession->setLastOrderStatus($order->getStatus());

    $this->eventManager->dispatch('checkout_submit_all_after', ['orders' => $orders, 'quote' => $quote]);

}
    /**
     * the API declaration and interface describe this function returning int, you can't return an array.
     * in order to do that you will have to create a new end point for that.
     */
    return;
}

How can I set grand total and subtotal of each order after order split?