If the vendor has a W-9 on file with you, the presumption is that this is a U.S. entity, not a Canadian one. (In that case, you would have a W-8 on file instead.)
If the vendor is a legitimate U.S. entity, you would not issue a 1042-S, but possibly a 1099, though if the vendor is a corporation, they are exempt from 1099 reporting (except in the case of legal or medical services).
A good rule of thumb: a W-9 generates a 1099; a W-8 generates a 1042-S (corporations not excepted). The remit-to address is immaterial here. This was confirmed with IRS reporting specialist Ms. Oroke (#1000269880).
If you have reason to doubt the vendor, you might verify whether the address on the W-9 is a U.S. one, and check the name/TIN match through the IRS TIN Match service (a best practice in any case). But it appears that the vendor understands the need to establish that it is a U.S. entity by submitting the W-9, even though as a corporation it is not subject to reporting.