Chinh Chu,
I think you are correct. For me, as a 2nd Lg VI speaker, "Đại diện" means "representing" and "thay mặt" means "on behalf of". They are the same meaning and seem 'strange' when used together in VI ... but of course I am a native EN speaker so may not totally hiểu!
If I see "Đại diện" in a signature block, it is "Represented by". If I see "Thay mặt", it is "On behalf of". If I see them together, only then is it "For and on behalf of".
Could this be another example where a VI writer is use synonymic repetition for reinforcement of meaning?
Cheers,
by and not on behalf of,
Shane
www.translingualexpress.com