Hello Luca,
as far as I can see is the size of a record from the tables VBAP and KNA1 greater than 512 characters. With RFC_READ_TABLE it is not possible to read a record which is larger than 512 characters. I think this could be the reason for your problem.
Unfortunetaly on my test system the tables VBAP and KNA1 doesn't exists. So I can't check the successful function of my HTA program. You find the HTA program in my Knowledge Collection here. Scroll down until "Access to SAP via ActiveX control", download the source as HTA file and execute it. Fill the form with your specific data and the table Name VBAP resp. KNA1 and press the button "Get data". Now you should see the content of the table. If it works you can look at the sub procedure GetData how I realize it and you can do it equally.
Otherwise try to reduce the fields of RFC_READ_TABLE with VBAP and KNA1 with the FIELDS table.
Let us know the results.
Cheers
Stefan