Try this:
SELECT
C.TradeName as CustomerName,
C.LegalName as LegalName,
(CASE WHEN CustomerName IS NULL OR DATALENGTH(CustomerName) = 0
THEN LegalName
ELSE CustomerName END) AS CustName
FROM
v_RptCompany_v001 c
INNER JOIN
v_RptAppCompany_v001 ac ON ac.PartyID = c.PartyID AND ac.BeginDate = c.BeginDate