Greetings, Jürgen:
My apologies for taking this long to complete this case. After retaking this issue, I end up realizing that for this specific LSMW Object type, every field containing no data should have been populated with a "/" character.
This is what I did and ended up uploading all my Vendor Master records.
I still do not know why this is a requirement for Vendor Master: other Direct Input objects do not require it.
Thanks a lot.