I can see this is your first time posting so I'd like to welcome you to the Community. Allow me to help share some details about auto-filling the vendor's name in QuickBooks Online.
QuickBooks Online, will auto-fill the vendor's name by typing the first 3-4 letters if there is only one vendor using those letters. However, if there are more that one vendor who has the same letters (example: Alice1, Alice Corp. Alice2) you'll need to select the specific name of the vendor whom you'd like to create the transaction for.
I can see how this feature would be helpful for you and your business. I encourage you to submit feedback, which is sent to the Product Development Team for review when determining features to be added in updates. You can do this from within QuickBooks Online by selecting the Gear icon and clicking Feedback or by going to the following link: QuickBooks Online Feature Requests.
You've got me here if there's anything else you need. I'll be around to help you out. Have a great weekend!