I have a list that has a vLookup to a large library with over 5000 items. I have overcome the large list with indexes and everything appears to be fine, however, the client has the page loaded into Teams using a webpage component.
When you go to the vLookup new items added do not show, they do show in raw SharePoint.
Do you mean that items added to the library from within teams does not show in the vLookup? – if so, please verify that the field you use to connect the items to the parent list (like _vLookupID > _vLookupParentID) has been filled in.