It is very rare to find Vishnu being addressed as Iswara.

He is the Sustainer of the Universe.

He is the supporter.

Vedic thoughts revolve around the Worship of Agni,Fire.

Though Agni carries what is offered in the homa to respective deities to whom the Oblations are offered,Agni is not addressed as Iswara.

That epithet is reserved for lord Vishnu.

The mahamantra Vishnu Sahasranama states thus.

‘

Yagyo Yagya-Patir-Yajva Yagyango Yagya-Vahanah

Yagyabhrud Yagyakrud Yagyi Yagyabhrug Yagyasadhanah

Yagyanantakrud Yagyaguhyam Annam Annada Eva Cha,

You are the Sacrifice,Yagnya.

Are the Lord of Yagnya,

You are the reason for Yagnya,

You are the Limbs of Yagnya,

Yagnya is your vehicle,

Yagnya is embedded in you,

You are the reason,cause for Yagnya,

You are the instrument of Yagnya,

You are the secret of Yagnya.

There is a difference between Yagnya and Yaaga.

Yagnya is a duty to be performed as it ought to be, as in the Categorical Imperative of Immanuel Kant,Yaaga is performed with a specific purpose in mind.

Sri Rudra addressed to Lord Shiva elevates Yagnya to another level.

While performance of Yagnya is to be without any specific desire in mind,it is hard to do so.

So Sri Rudra dedicates Yagnya thus.

Vaak Yagnyena kalpathaam,

Mano Yagnyena kalpathaam,

Chakshur Yagnyena kalpathaam,

Srothram Yagnyena kalpathaam,

Atma Yagnyena kalpathaam,

Yagnyo Yagnyena kalpathaam.

After offering the beneficial results that accrue by performing Yagnya ( though we may not seek the fruits of Yagnya,they will accrue),to Speech, Mind,Eyes,Ears,Self,Sri Rudra dedicates the fruits of Yagnya to Yagnya Itself!

Such an Yagnya has a deity.

That is Yagnyena,a Manvantara Avatar of Vishnu .

Yagneswara.

Yajna (Sanskrit: यज्ञ, yajña) or Yajneshwara(“Lord of Yajna”) is mentioned as an avatar of the Hindu god Vishnu in the Bhagavata Purana. As Yajna, Vishnu is the embodiment of the Hindu sacrifice ritual or Yajna.He was also Indra (king of the gods) of the Svayambhuva Manvantara, era of Svayambhuva Manu...

‘
Yajna is classified as one of the 14 main Manvantara-avatars (an avatar corresponding to a Manvantara and who supports the corresponding Indra and other gods to maintain the principles of religion) called vaibhava-avatars. Yajna is also categorized as a Kalpa-avatar (an avatar corresponding to an aeon called Kalpa) of Vishnu.- Devi Bhagavatha,Garuda Puranas’

‘

Yajna had a twin sister named Dakshina (“donation”). Later, Yajna married Dakshina and had twelve sons. These twelve Devas (gods) are collectively called the Yāmas.

The Bhagavata Purana identifies Yajna with Vishnu and Dakshina with goddess Lakshmi, the goddess of fortune and consort of Vishnu. After Yajna’s birth, he lived at the house of his grandfather Svayambhuva Manu. The sons to Yajna and Dakshina are named as Tosha, Pratosha, Santosha, Bhadra, Sânti, Idaspati, Idhma, Kavi, Vibhu, Svahna, Sudeva and Rocana. They are collectively called as the Tushita gods. Later Yajna is described to become the Indra. Garuda Purana says that he performed many sacrifices’ – Vishnu Purana,Bhagavatha Purana.

Source for block quotes.

https://en.m.wikipedia.org/wiki/Yajna_(avatar)

The details of Dakshina and children of Yagnyena have philosophical import.

Shall write.