The uniqueness of the temple has just begun, when you come round the sanctum sanctorum, right behind you would find a small entrance and a flight of stairs which would lead you to another sannidhi, in first floor, where Lord is in kidhandha thirukolam (sleeping posture) with Sridevi and Bhudevi. When you go to the next level (second floor) you have Lord as Paravasudevan in nindra thirukolam (standing posture). Archanai to the Ranganathar in the first floor are done only on ekadasi as that is only day you could have access to view both Ranganathar and Paravasudevan. Archanai is not performed for Paravasudevar. The picture below is Paravasudevar as seen on Srirangam vimanam.
Legend goes thus: Dwara Balagas (gate keepers of the Lord) were born as sons to King Veerochana, who prayed the Lord for children. Villavan and Pallavan (Dwara balagas) performed aswamedha yagam, pleased with their prayers, Lord appeared in form of Narayanam in Irundha thirukolam (sitting posture). As the Lord gave his seva as Paramapadha – Sri vaikundanathan, this divya desam is known as Vaikunda Perumal kovil.
The vimanam is called Mukunda vimanam but since it is constructed as Ashtanga vimanam, it is included among temples with Ashtanga vimanam.