Welcome to the QuickBooks Community. Allow me to offer some insight into how Price Rules work. In QuickBooks Online, Price Rules are available exclusively for customers. It has been brought to our attention that recently a Price Rules line is showing on the Vendor Detail screen as you mentioned. What I've done for you is submit feedback to our team of engineers to have this corrected. This will be reviewed by them and may be changed in a future update.
If you're curious about Price Rules, I've included steps down below for how to turn on Price rules and create them for customers:
How to turn on Price Rules:
Select the Gear icon at the top, then choose Account and Settings (Company Settings).
Choose Sales from the left menu.
Click the Pencil icon on the right of Products and services.
Put a checkmark in the box of Turn on Price Rules Beta.
Choose Save, then select Done at the bottom right.
How to create a Price Rule:
Select the Gear icon at the top, then choose All lists.
Choose Price Rules.
Click New price rule on the upper right.
For more on Price Rules, please take a look at this FAQ article: Price Rules FAQs.
As I mentioned earlier, I've submitted this to the engineering team and it may be corrected in a later update. I hope this helps to clear the air on the subject. However, if you have any questions about this or anything else QuickBooks, don't hesitate to ask me here in the Community. I'll be here if you need anything else.