Matrix Table not giving the correct total amount

It would have been easier for us to provide a solution had you provided a sample pbix (confidential data removed). But just given what's available, here's what I would try: Keep a separate measure for vendor level only. Create another measure to return the total

TotalSellPrice_BillsReceived - Vendor and Total =
IF (
    SELECTEDVALUE ( Vendor_Template[Vendor Reference] ) = "Total",
    SUMX (
        VALUES ( VendorProjectType[Manufacturer] ),
        [TotalSellPrice_BillsReceived - vendor]
    ),
    [TotalSellPrice_BillsReceived]
)