Hello there, @261. I appreciate you for joining here.

It could be a browser-related issue, or their status were accidentally marked as inactive in your QuickBooks Online account. Now, let's perform some steps to view the missing vendors in your QuickBooks.
 

First, let's check the vendor status:

1. Go to the Expenses menu.
2. Choose Vendor. 
3. Click the small gear icon beside the Printer icon. 
4. Check the Include Inactive under Other. 
5. Select Make active to re-show the name in the list. 

 I'd also recommend utilizing the audit log feature to review the company activities. On the other hand, perform these basic troubleshooting steps.

Step 1: Use an incognito window:

It's a way to check issues caused by a full cache.

• Google Chrome: Ctrl + Shift + N
• Safari 11 or newer: ⌘ + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P

Step 2: Delete cache and temporary internet files:

If it works, clear the cache to optimize the browser's performance. Another way is to use compatible browsers with QuickBooks Online. 

I've also added articles about handling duplicate vendor accounts, reports, and transactions. 

• Merge duplicate accounts, customers, and vendors in QuickBooks Online
• Reports
• Expenses and vendors.

Please let me know how these steps go. If you need further assistance, comment below. Bye for now, stay safe always.