Both of these phones come at a similar price and provide almost similar specifications and they are not preferrable with some specifications as well.
So, it is hard to choose between them and it is all user's choice as to what specifications is he/she looking for.
Let's see some specifications-
Processor: The Vivo Y81 comes with the MediaTek Helio P22 processor which is better than the HiSilicon Kirin 659 processor of Honor 9n.
Camera: Both phones are moderately good with the camera with having the same aperture of F2.2. In Honor 9N, however, you will get a better front camera of 16 MP in comparison to 5 MP of the Vivo Y81.
Display: The display of Honor 9n is much better than that of Vivo Y81 as it has a pixel density 432 ppi of in comparison to 270 ppi.
Check their full comparison Honor 9N and Vivo Y81