I have a sharepoint list that requires a user to input a part number (text field) and a vendor (lookup field) along with some other fields. I want to allow the user to enter duplicates of either the part number or vendor (so enforce unique values will not work), but not allow duplicates of the combination of the part number and vendor. In other words, I want to allow only one part number entry per vendor and alert the user if the part number has already been entered for that vendor. Is there a way to accomplish this with a validation rule or basic functionality?

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There is no OOB solution for this requirement, since you will need to compare the input twice in order and with an unknown collection from the column values. But there are several workarounds based on which version of SharePoint you are using.

For example, you can create Event Receiver (custom solution) if you are on SharePoint on-premises, or use InfoPath to build validation on field controls.

Here is a post for your reference: Validating between two columns to be unique.

If you are using SharePoint Online, you can use PowerApps to do the validation.

References: How to check duplicate item in list using powerapps. / Identifying Duplicate Entries.

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