All of my vendors suddenly are no longer in my mobile app. I can see them online. How do I get them back in mobile without adding them manually?

I can see how hard you tried here. No worries, I'll make it up to you to ensure you never have to deal with this again. 

The QuickBooks Online mobile app is always in sync with the web version. The tasks and changes you've made in the app should show up on the web, and vice versa. I'd provide you with a few troubleshooting steps to check if this is a mobile application issue. Let's start by resetting the data in your mobile app. As long as you’re connected to the internet, you should be able to update your data. Here’s how to do it on different mobile operating systems:

iOS

1. Go to Menu ☰ and tap Help & Feedback.
2. Tap Refresh Data.
3. Note: A message appears “Caution. If there was a problem getting your latest QuickBooks Online data, you can refresh it now. It may take a few minutes.”
4. Tap Refresh to confirm.

Android

1. Go to Menu ☰ then tap More Options ⋮.
2. Tap Settings then Refresh Data.
3. Tap YES to confirm.

Once done, you can close and reopen the mobile app and check for the vendors. Also, I'd suggest restarting your phone. If you continue to get the same result, uninstall and reinstall the application to start fresh. Follow the steps below to delete it.

1. Tap and hold QuickBooks Online on your phone.
2. Drag the app to the Uninstall section.
3. Select Yes to confirm.

You can also check this link to learn more about the features to use while using the QBO app: Compare mobile app features.

I've always got your back whenever you have other issues when doing tasks in the mobile app. Drop your reply anytime and I'll get back to you. Have a great day ahead.