What is the position of Lord Vishnu as per Shiva Purana? Is he given the status of Supreme anywhere in Shiva Purana?
Lord Vishnu is the Form of Para-Brahman that preseves the world according to the Shiva-Purana:
a-kAras cha mahAvijam rajah srastrA chaturmukhah/u-kArah prakritir yonih sattvam pAlayitA harih,ma-karo purusho vijam tamah-samhArako harah// (Vayaviya-samhita, Uttara bhaga, chapter 7).
Meaning : [Of A-U-M], A is the great 'vija' of the fourheaded Brahmaa Who embodies rajo-guna and is the creator, U is the vija of Hari Who embodies sattva-guna and preserves the world by His prakriti and is the source of everything, and M is the the vija of Hara Who embodies tamo-guna and is the destroyer by His Paurusha.
In another place of the Shiva-Purana we get
A-kAro brahma iti Aha U-kAro vishnur uchchyate/Ma-karas tu UmA jneyA prashantam shAshvatam dhruvam// (sanatkumara-samhita)
Meaning : A-kara is Brahmaa, U-kara is Vishnu and M-kara is Uma. All are very gentle, Eternal and Truth.
It is interesting to note that the Vishnu-Purana also says the same:
shaktayo yasya devasya brahmaa-vishnu-shivAtmikah/brahmaa-vishnu-shivAh Brahman pradhAnA brahmashaktayah//
Meaning : The three Principal forms of Brahman in form of Its energy are Brahmaa, Vishnu and Shiva.
sarga-sthiti-anta-kArinim brahma-vishnu-shivAtmikAm/sa sanjam yAti bhagavAn eka eva janardanah//
Meaning : The three forms of the Same Bhagavan Janardana are Brahmaa, Vsihnu and Shiva for creation-preservation-destruction. Brahman becomes Manifest in these three forms.
Reference: Pranava-Prema-Piyusha, Mahamilan Math, Vol.1, pages 135-141