<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="catalog_product_save_after">
        <observer name="test_name" instance="YOUR_NAMESPACEYOUR_MODULENAMEObserverProductsaveafter" />
    </event>
</config>

<?php

namespace YOURNAMESPACEYOURMODULENAMEObserver;

use MagentoFrameworkEventObserverInterface;

class Productsaveafter implements ObserverInterface
{
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $product = $observer->getEvent()->getProduct();
        $id = $product->getId(); //Get Product Id

        //Get Quantity
        $stockItem = $product->getExtensionAttributes()->getStockItem();
        $stockData = $stockItem->getQty();
        // Get new Qty
        $_vendor_qty = $product->getVendorQty();
        $_on_hand_qty = $product->getOnHandQty();
        $totalQty = $_vendor_qty+$_on_hand_qty; //Add New Qty


        $stockItem->setQty($totalQty); //Set New Qty to Main Qty
        $stockItem->save();

    }
}