It's an Access database (I know, bad idea, but it's just a proof of concept right now, if the project is approved we will use Oracle), but I fear you are correct in that the ASP .NET sql parser is not fully ISO 8859 compliant.
Oddly, Access does not appear to require the trailing ";" Here's the SQL that works:
SELECT
POHeader.PONumberPK,
Vendors.Name,
POHeader.ItemTotal,
POHeader.[Date],
Vendors.VendorPK
FROM
(
Vendors INNER JOIN
POHeader
ON
Vendors.VendorPK = POHeader.VendorFK
)
INNER JOIN
ApprovalRequired
ON
POHeader.PONumberPK = ApprovalRequired.PONumber