Is Lord Mahavishnu a form of Aadi parashakthi ?

Also, according to Vayaviya Samhita of Shiva Mahapurana Lord Shiva alone is Purusha while all others, including Brahma, Vishnu and Rudra are Prakriti Amshas. 

As per Shakta Mahabhagavatam, Srimad Dakshina Kalika Herself incarnates in Lohita Kalpa as Vasudeva Sri Krishna. 

Lord Krishna

Todala Tantra equates Dasa Mahavidyas with Dasa Avataras and it is said that Lord Vishnu's ten Avataras are actually Amshas of Dasa Mahavidya, and Lord Krishna is born as Amsha of Adi Mahavidya Bhagavati Dakshina Kalika. 

Dasha Maha vidyas

Similar to this, Lalita Sahasranama Stotram and Lalitopakhyanam section of Brahmanda Purana invoke Lalitambika as 'karAngulI nakhotpannA nArAyanasya daSAkruti' (one who creates Narayana's ten Avatars from Her finger nails).

Lalitambika

Also, Klim is Kama Bija, and it is said in Dakini Tantra that personification of Klim Bija is Kameshvari (Lalita Tripurasundari). She then becomes Krim Bija (Samhara Bija) and is seen as Supreme Kalika. In order to make the co existence of Krim & Klim possible, Kameshvari also becomes Krishna. 

This is emphasized more in Tantraraja Tantra, where Lalitambika Herself transforms into Lord Krishna and sports with Gopikas.
Remember, Klim Bija denotes Kameshvari and the same Bijakshara is also used in Krishna Mantras. E.g; om klIm krShNAya govindAya gopIjana vallabhAya. Why Bijakshara of Lalitambika be used in Krishna Mantra unless both are one?.

A reference from Shakta Mahabhagavatam which  talks about the Kalika taking avatar as Lord Krishna.

After Mahapralaya had occurred, to receive the seeds of creation, Adya Kali indulged in reverse-copulation with Lord Mahakala. Their bed was the corpses of Sadashiva, Kameshvara, Rudra, Vishnu & Brahma laid atop one another, placed on a burning cremation pyre amidst the Mahasmashana Mandala.

After dallying to their heart's content, Lord Mahakala said "Parameshani! I wonder what do you feel when you sit on top of me, throbbing in the pleasure of Viparita Rati. I am your devotee eternally. Oh Kalike! Have mercy on me, and bestow this boon upon me that I may get opportunity to feel same way as you."

Adya Kali said "Oh Parameshvara! Thou who governs as Sadashiva, deludes as Kameshvara, creates as Brahma, sustains as Vishnu and finally annihilates as Rudra at my behest! Thou have spoken of my heartiest wish. My Lord! I am always thine servant forever.

 For my selfless love for thou, I must take up a masculine form by forsaking my Yoni in Dvapara Yuga, at thine behest. I must take two-fold incarnations; one as Govinda, the son of Devaki; another as Ekanamsha, the daughter of Yashoda. Oh my beloved husband! Be gracious upon me by incarnating as Raseshvari, the daughter of Vrishabhanu. Let the Eight Bhairavas be the eight Bharyas and Eight Matrikas be the 16000 wives. Let Vishnu be Balabhadra and let the fifteen Nityas be Gopikas."

hereafter, by placing Her Yoni at Kamarupa and took two-fold incarnations of Krishna and Kaushiki. Krishna emerged from the womb of Devaki and was raised by Yashoda to slay Kamsa. On the other hand Kaushiki manifested from the outer sheath of Parvati and emerged from the womb of Yashoda to slay Sumbha and Nisumbha.

To accompany Adya Kali's Krishna Avatar.

“rudraatpravartate biijaM biijayonirjanaardanaH |” (Rudra Hridayopanishad 8)

“Rudra is the generator of the seed. Vishnu is the embryo of the seed

Then RV 7:40:5 says “Rudra derives his strength from Vishnu”.

Rudra derives strength from Vishnu as he is his Shakthi. From which we again understand that Vishnu and Shakthi are the same. Vishnu is the male form of Aadi Shakthi.