Well, communication has to be bidirectional if it is communication at all.
Otherwise you are just flooding their interface with packets they most likely drop. Reminds of a DoS attack.
With a vpn tunnel - you can encrypt packets all you want, if the other side doesn't know how to decrypt them they will get dropped. It is not a one-way tunnel, it is a waste of bandwidth
For a one way tunnl to exist, you have to configure both hosts correctly, and then use iptables or routing to make sure only one side can send traffic through.