How about having the vendor do somwthing like this ...

$ touch file_to_be_sent.end
$ ftp your.server.com
...
> put file_to_be_sent.dat
> put file_to_be_sent.end
> bye
[download]

This way you will know file_to_be_sent.dat has completed uploading when file_to_be_sent.end appears on your file system.