Argthtjtr

"Having thus spoken to Nidāgha, the
Brāhmaṇa Ṛbhu went away, leaving his
disciple profoundly impressed, by his
instructions, with belief in unity. He beheld all
beings thenceforth as the same with himself
and, perfect in holy knowledge, obtained final
liberation. “In like manner do you, oh king,
who knows what duty is, regarding equally
friend or foe, consider yourself as one with all
that exists in the world. Even as the same sky
is apparently diversified as white or blue, so
Soul which is in truth but one, appears to
erroneous vision distinct in different persons." (Vishnu Purana, Book 2, Chapter 16)

The above passage, seems to show that the plurality of all Jivas is unreal and that all Jivatmas are in essential, one Atman in reality.

This more or less seems to be in agreement with Advaita Vedanta and direct contradiction with Vishishtadvaita Vedanta, which states that all Jivas are not one, but multiple (atleast superficially).

So my question is how do Vishishtadvaitins interpret this passage? How do they show the above passage does not indicate oneness of Jivatmas?

We find in the Sri Bhashya introduction the explanation of how to interpret such passages from the Vishnu Purana.

Acharya Ramanuja writes here in George Thibaut's translation

Also the sloka beginning 'Owing to the difference of the holes of the flute' (Vi. Pu. II, 14, 32) only declares that the inequality of the different Selfs is owing not to their essential nature, but to their dwelling in different material bodies; and does not teach the oneness of all Selfs. The different portions of air, again, passing through the different holes of the flute--to which the many Selfs are compared--are not said to be one but only to be equal in character; they are one in character in so far as all of them are of the nature of air, while the different names of the successive notes of the musical scale are applied to them because they pass out by the different holes of the instrument. For an analogous reason the several Selfs are denominated by different names, viz. gods and so on. Those material things also which are parts of the substance fire, or water, or earth, are one in so far only as they consist of one kind of substance; but are not absolutely one; those different portions of air, therefore, which constitute the notes of the scale are likewise not absolutely one. Where the Purâna further says 'He (or "that") I am and thou art He (or "that"); all this universe that has Self for its true nature is He (or "that"); abandon the error of distinction' (Vi. Pu. II, 16, 23); the word 'that' refers to the intelligent character mentioned previously which is common to all Selfs, and the co-ordination stated in the two clauses therefore intimates that intelligence is the character of the beings denoted 'I' and 'Thou'; 'abandon therefore,' the text goes on to say, 'the illusion that the difference of outward form, divine and so on, causes a corresponding difference in the Selfs.' If this explanation were not accepted (but absolute non-difference insisted upon) there would be no room for the references to difference which the passages quoted manifestly contain. Accordingly the text goes on to say that the king acted on the instruction he had received, 'he abandoned the view of difference, having recognised the Real.'--But on what ground do we arrive at this decision (viz. that the passage under discussion is not meant to teach absolute non-duality)?--On the ground, we reply, that the proper topic of the whole section is to teach the distinction of the Self and the body--for this is evident from what is said in an early part of the section, 'as the body of man, characterised by hands, feet, and the like,' &c. (Vi. Pu. II, 13, 85).--For analogous reasons the sloka 'When that knowledge which gives rise to distinction' &c. (Vi. Pu. VI, 7, 94) teaches neither the essential unity of all Selfs nor the oneness of the individual Self and the highest Self