You can't have two different junction paths.
Why do you need to have the path be /mnt/vol1? What's the use case for that? In NFS, all you'd need to do is mount with the following command:
#mkdir /mnt/vol1
# mount ontap:/vol1 /mnt/vol1
And then your client would access the storage from /mnt/vol1.
The reason you can't simply make /mnt/vol1 the junction path is that /mnt has to exist in ONTAP's namespace. When you created the volume with /mnt as the path, that allowed you to mount to that path within ONTAP.
The reason the SMB share failed after that is you likely didn't create a share with /mnt/vol1 as the path.
But ultimately it shouldn't matter what the path is inside ONTAP; what matters is what the client is presented.
For SMB, it would be \\ontap\sharename regardless of which path that share points to in ONTAP.
For NFS, it would be /vol1 or /mnt/vol1. But it doesn't make sense to create the junction path /mnt/vol1 when you can just create that path on the client if you need that specific path.