If you're using QuickBooks Bill Pay for QuickBooks Online, your vendor will automatically receive two notifications. The first email confirms that you have scheduled a payment for them. The second email notifies them when to expect the payment. For reference, you can visit this article: Learn about QuickBooks Bill Pay.
However, if you're not, you can consider using a third-party app to achieve it. Let me assist you in finding one.
On the left navigation panel, go to Apps.
Select the Find apps tab.
Look for apps using the search tab or the Browse categories dropdown menu.
For better management of your bills, you can check out these informative articles: