With datatable, the 2nd object references here weren't displaying in the view. The entire column was blank. 
   

     @track columns = [
            {label: 'Vendor', fieldName: 'Vendor_Name__r.Name', sortable: true},
            {label: 'Quote Name', fieldName: 'Name', sortable: true},
            {label: 'Status', fieldName: 'Status__c', sortable: true},
            {label: 'Purchase Agreement', fieldName: 'Purchase_Agreement__r.Name', sortable: true}
        ];
[![enter image description here][1]][1]

When I change Vendor_Name__r.Name TO Vendor_Name__c AND Purchase_Agreement__r.Name to Purchase_Agreement__c, the Ids render in the view just fine.
 

    @track columns = [
            {label: 'Vendor', fieldName: 'Vendor_Name__r.Name', sortable: true},
            {label: 'Quote Name', fieldName: 'Name', sortable: true},
            {label: 'Status', fieldName: 'Status__c', sortable: true},
            {label: 'Purchase Agreement', fieldName: 'Purchase_Agreement__r.Name', sortable: true}
        ];

[![enter image description here][2]][2]

As a workaround, I figured the following would work, but all rows in my datatable are now blank. In the console, it does print this out:

    ***** quote: {"Vendor_Name__c":"0013k00002io4VgAAI","Name":"0029","Status__c":"Ready for Review","Purchase_Agreement__c":"a4vDL0000013VfjYAE","Id":"a4cDL000000hddsYAA","Vendor_Name__r":{"Name":"4 BEST BUSINESS CORP","Id":"0013k00002io4VgAAI"},"Purchase_Agreement__r":{"Name":"PA# 0001","Id":"a4vDL0000013VfjYAE"}}


HTML

    <template if:true={quoteList}>
        <lightning-datatable 
            data={quoteList} 
            columns={columns} 
            key-field="Id"            
            default-sort-direction={defaultSortDirection}
            sorted-direction={sortDirection}
            sorted-by={sortedBy}
            onsort={handleSort}>
        </lightning-datatable>
    </template>


JS

        @track columns = [
            {label: 'Vendor', fieldName: 'VendorName', sortable: true},
            {label: 'Status', fieldName: 'Status', sortable: true},
            {label: 'Quote Name', fieldName: 'QuoteName', sortable: true},
            {label: 'Purchase Agreement', fieldName: 'PurchaseAgreementName', sortable: true}
        ];


        connectedCallback() {
            getQuotes({rfqID : this.recordId, statusValue : this.filterValue})
            .then(results => {
                // this.quoteList = results;
                
                results.forEach(result => {
                    console.log('**** VendorName: '+result.Vendor_Name__r.Name);
                    this.quoteList.push({
                        VendorName: result.Vendor_Name__r.Name,
                        Status: result.Status__c,
                        QuoteName: result.Name,
                        PurchaseAgreementName: result.Purchase_Agreement__r.Name
                    });
                })

                this.quoteList.forEach(
                    quote => console.log('***** quote: ' + JSON.stringify(quote))
                );


                this.error = undefined;
            })
            .catch(error => {
                this.error = error;
                this.queryResults = undefined;
            });
    }


  [1]: https://i.sstatic.net/fMnVo.png
  [2]: https://i.sstatic.net/NJq9g.png