Dera Satyaveer Singh, it may not answer your question in full, but attached please find an article describing some of the properties of thionyl chloride.
Dear Satyaveer Singh, sorry to hear that the reference wasn't really useful. Even a SciFinder search for the topic "dew point of thionyl chloride" revealed no useful results. 😥
Dear Satyaveer Singh, I join Prof. Frank T. Edelmann answers, and I suggest you measure it by your self if the necessary lab facilities are available. The following RG link deals with an approch to do that. My Regards