Hello,
I have a lot of vRO endpoint in my case, all in v7.5.
To be able to add it as an endpoint, you need to attribute the administrative right ( on vRO) to your user ( used for the connexion from lifecycle) inside the authentication provider, inside the admin group , you must add your user or group ( with your user part of) inside.
To be sure that your user used for Lifecycle connexion have the good right, you can test it on the Test login tab.
After that you can add your VRO as endpoint, if it's still KO, please make sure that a curl command from your lifecycle to your vRO on port 8281 work fine.
Regards,