Unless I'm mistaken, even if there is a default vendor assigned in CI_Item, if the ""vendor number"" field on the sales order line does not have a vendor assigned, that line will not generate a line on a PO.
Assuming that is true (I did not test that this morning) :
1. Work around would be to simply blank out the vendor number field on some Sales Order lines as you populate the line.
2. Use a UDF in CI_Item like @NancyHanson suggested for the items that should not create a PO. Pull that on to the SO Line detail. Then add script during SO Line Entry to blank out the vendor field. This script could fire on post validation of the item code. Or you could add a button on the SO lines tab that the user could click on. The script could then read through each line and blank out the field based on the value in the UDF.