Title says all, this is what error I get and what code I'm using. The class has few vectors and quaternions, the vectors are ok but quaternions aren't.

JsonSerializationException: Self referencing loop detected for property 'normalized' with type 'UnityEngine.Quaternion'

arena1 = JsonConvert.DeserializeObject<Arena>(Interface.Oxide.DataFileSystem.ReadObject<string>(fileNameArena1),
                        new JsonSerializerSettings()
                        { 
                            ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
                        });

You can save vectors and get quaternions from them like Quaternion.Euler(vector)

But I want to save Quaternions, not a work around for it...

ReadObject already does DeserializeObject, just put Arena as a type instead of string.
You could create a DynamicConfigFile and set the needed JsonSerializerSettings in Settings variable. And use ReadObject method.