First of all What is a Yajna or Sacrifice.?
Satapatha Brahmana (9:1:2:11) says.:
“bāhyenāgniṃ harati | ime vai lokā eṣo 'gnirebhyastallokebhyo bahirdhā śucaṃ dadhāti | bahirvedīyaṃ vai vedirasyai tadbahirdhā śucaṃ dadhāti |”,
“Outside the fire-altar he throws it; for this fire-altar indeed is the same as these (three) worlds: he thus puts pain outside these worlds;--outside the sacrificial ground (vedi); for the Vedi is this (earth): he thus puts pain outside this earth”.
So, a Sacrificial Alter is the 3 world itself with Vedi as the earth.
Let’s see what Vedas state about the Earth.
Agni produced the Earth and he became the Earth in fact. So, Earth is same as Agni. This is evident from the below verse from Vedas.:
“tā vā etā nava sṛṣṭayaḥ | iyamasṛjyata tasmādāhustrivṛdagniritīyaṃ hyagnirasyai hi
sarvo 'gniścīyate |” (Satapatha Brahmana 6.1.1.14).
“This (earth), then, was created as (consisting of) these same nine creations. Hence they say, 'Threefold (three times three) is Agni;' for Agni is this (earth), since thereof the whole Agni (fire-altar) is constructed”.
Then that Earth sang (gâ), therefore she is Gâyatrî. But infact it was Agni who sang. Hence it is Agni who is Gâyatra.
"abhūdvā iyam pratiṣṭheti | tadbhūmirabhavattāmaprathayatsā pṛthivyabhavatseyaṃ
sarvā kṛtsnā manyamānāgāyadyadagāyattasmādiyaṃ gāyatryatho āhuragnirevāsyai
pṛṣṭhe sarvaḥ kṛtsno manyamāno 'gāyadyadagāyattasmādagnirgāyatra iti tasmādu
haitadyaḥ sarvaḥ kṛtsno manyate gāyati vaiva gīte vā ramate |" (Shatapatha Brahmana 6.1.1.15).
“'This (earth) has indeed become (bhû) a foundation!' (he thought): hence it became the earth (bhûmi). He spread it out (prath), and it became the broad one (or earth, prithivî). And she (the earth), thinking herself quite perfect, sang; and inasmuch as she sang (gâ), therefore she is Gâyatrî. But they also say, 'It was Agni, indeed, on her (the earth's) back, who thinking himself quite perfect, sang; and inasmuch as he sang (gâ), therefore Agni is Gâyatra.' And hence whosoever thinks himself quite perfect, either sings or delights in song".
Therefore Gayatri is none other than Agni indeed. It means Agni (male god) is visualized as Gayatri (the female deity). But indeed they are same.
“agnirvai gāyatrī |” (Shatapatha Brahmana 3:4:1:9).
“Gâyatrî is verily Agni”.
Therefore, Agni is below because agni is Earth itself and so is near vedi or the vedi itself.
So, Rig Veda (10:5:7). states Agni as Androgynous as,:
“aghnirha naḥ pra thamajā ṛtasya pūrva āyuni vṛṣabhaścadhenuḥ ”,
“Agni, our first-born of Holy Order (rita), the Milch-cow and the Bull in life's beginning.”
Now, Rig Veda 1.22.20.:
तद्विष्णोः परमं पदं सदा पश्यन्ति सूरयः । दिवीव चक्षुराततम् ॥
“The wise ever contemplate that supreme station of Viṣṇu, as the eye ranges over the sky.”
Commentary by Sāyaṇa: Ṛgveda-bhāṣya
paramam padam = supreme degree or station, svarga.
Meaning of Padam.:
padaṃ < padam < pada
[noun], accusative, singular, neuter
“word; location; foot; footprint; pada [word]; verse; footstep; metrical foot; situation; dwelling; state; step; mark; position; trace; construction; animal foot; way; moment; social station; topographic point; path; residence; site; topic.”
So, from the above verse, commentry and meaning by Sayana we can conclude that Vishnu's Param Padam is Swarga and it is also identical with him as his own seat or position.
Thus, Vishnu is the upper portion being Heaven.
Again, Yajur Veda 2.8.:
वसुमतीमग्ने ते छायामुप स्थेषं विष्णो स्थानमसि ।
Oh Agni, I have placed the earth under your shadow, and you are the supreme abode of Lord Viṣṇu.
Shatapatha Brahmana (3.1.3.1) calls Agni as “agnirvai sarvā devatā ” which means, “Agni is all the deities”
“tvamaghna indro vṛṣabhaḥ satāmasi tvaṃ viṣṇururughāyo namasyaḥ
tvaṃ brahmā rayivid brahmaṇas pate tvaṃ vidhartaḥsacase purandhyā ” (RV 2.1.3).
"Hero of Heroes, Agni! Thou art Indra, thou art Viṣṇu of the Mighty Stride, adorable: Thou, Brahmaṇaspati, the Brahman finding wealth: thou, O Sustainer, with thy wisdom tendest us".
“aghne yaṃ yajñamadhvaraṃ viśvataḥ paribhūrasi sa iddeveṣu ghachati ” (RV 1.1.4).
"Agni, the perfect sacrifice which thou encompassest about Verily goeth to all the Gods".
“tava praśāstraṃ tvamadhvarīyasi brahmā cāsi ghṛhapatiśca no dame ” (RV 2.1.2).
"Thou art Director, thou the ministering Priest: thou art the Brahman, Lord and Master in our home".
“rājantamadhvarāṇāṃ ghopāṃ ṛtasya dīdivim vardhamānaṃsve dame ” (RV 1.1.8).
“[Agni] Ruler of sacrifices, guard of Law eternal, radiant One, Increasing in thine own abode".
Thus, from the above verses we get that Agni is all the Gods, including, Vishnu. He is also the ruler, priest and the full perfect sacrifice himself as Agni makes the sacrifice complete by encompassing it fully, and that goes to all gods when it gets fulfilled in this manner. This implies that Agni is the fulfiller of sacrifice.
In this answer of mine, I have discussed how Agni takes 3 steps of Vishnu in different meters. As Agni is called 3 fold.
At last,
“atha viṣṇukramān kramate | devānvā eṣa prīṇāti yo yajata etena yajñena 'rgbhiriva
tvadyajurbhiriva tvadāhutibhiriva tvatsa devānprītvā teṣvapitvī bhavati teṣvapitvī
bhūtvā tānevābhiprakrāmati |” (Satapatha Brahmana 1:9:3:8).
“He (The sacrificer) now strides the (three) Vishnu-strides. He who sacrifices assuredly gratifies the gods. In gratifying the gods by that sacrifice--partly by riks, partly by yagus, partly by oblations (of samas)--he acquires a share among them; and having acquired a share among them, he goes to them”.
So, Agni along with the sacrificer takes 3 steps viz from Household Fire to Funeral Fire to Sacrificial Fire and Ultimately after engulfing all Devas including Vishnu reaches Brahman viz The All Exchanging Fire (Paramatma).
Bdw, remember these are all inner sacrifices done through Pranayama. See the above given link for more.
As for the ranking, Shiva Sankalpa Shuktam of Rig Veda Khilani gives the ranking as follows.:
Rig Veda Khilani SSS.:
परात्परतरो ब्रह्मा तत्परात्परतो हरिः । तत्परात्परतो ईश तन्मे मन शिवसंकल्पमस्तु ।। 18 ।।
Greater than the great is Brahma, greater still than that great one is Hari, even greater than this one is Ishana. May my mind always remains full of Auspicious Shiva thoughts.
I hope this clarifies all your queries. Prd..