funny enough, I just said to myself, I need to hear the two words side by side to learn to distinguish the difference... As duolingo does not offer such feature, I went to google translator, typed 'vin' and listened, then typed 'vinen' and listened. I get what you are saying, when I heard those two after one another, I heard 'viin' and 'viinn', which was OK.
The weird bit was when I tried to listen to the same word multiple times - write 'vin', click listen. click listen again... and the same with 'vinen' and the google's robot does this: 'vin' ---> viin, vin, viin, vin 'vinen' ---> viinn, viin, viinn, viin
so maybe duolingo's robot has the same bug and sometimes pronounces it wrong :)