I thought you had found the issue (might still be the issue though) but I´ve changed A1:A90 to number format and all other numbers in the Excel file to number format as well.
Strangely enough I still get the same error message, i.e.

     TESTO = Application.WorksheetFunction.Match("8077", Sheets("Sammanställning").Range("a1:a90"), 0)   'DOES WORK
              TESTO = Application.WorksheetFunction.Match(8077, Sheets("Sammanställning").Range("a1:a90"), 0)     'DOES NOT WORK
              TESTO = Application.WorksheetFunction.Match("Vendor", Sheets("Sammanställning").Range("a1:a90"), 0) 'DOES NOT WORK. Vendor is a variable with value 8077.
              TESTO = Application.WorksheetFunction.Match(Vendor, Sheets("Sammanställning").Range("a1:a90"), 0)   'DOES NOT WORK. Vendor is a variable with value 8077.