Narayan and laxmi missing....??

There are different opinions on Vaikunth was actually vacated during Ramavatar. Some claim all avatars are part manifestations, so Vishnu never actually left Vaikunth .

But if I have understood Valmiki Ramayan correctly, Vishnu did leave Vaikunth for his sojourn on Earth. In the Uttara Khanda during his conversation with Time / Kala, Kala given Rama Brahma's message, in which Brahma mentions that Vishnu had decided to stay on Earth for 11000 years - which had elapsed.

Rama too acknowledges that his work on Earth is over and he was keen to return to his abode.

Such a remark would have been redundant, had Vishnu never left Vaikunth.

Can't this convo mean that bramha was sending message for sagun form of The Lord to his original form the nirgun form?

Possible. The Nirgun form of the Lord stayed in Vaikunth, while the sagun form manifested itself on Earth. But even in that case, the serials cannot show Vishnu in Vaikunth / Ksheersagar and Rama on Earth at the same time.

And the universe was not without its preserver,as you say. The preserver was present on Earth in a human form.

Because I also remember in that conversation between Kaala and Rama, Brahma asks Rama to continue staying on Earth, if He wanted to protect the living beings for sometime more.

So The Lord is every where present in the nirgun form so he has to be present in vaikundha in nirgun form too. But yes the sagun form cannot be present in two places I guess

The Narayan & Mahadev avatars can't be equated, given that it was always a Rudransh that came up, whether it was Jalandhar, Hanuman or anyone else in Mahadev's case. Hanuman, for instance, was said to have been born of one of his limbs. In the case of Narayan's avatars, except Parashurama, Narayan completely took those forms, did his jobs and left.

Although Valmiki Ramayan says differently, I too find it very hard to digest that Vishnu simply left Vaikunta for his incarnations. Simple reason is that he did not need to. His mere ansh had more than enough power to destroy all evil. Personally, I don't believe even Valmiki wrote this. I strongly believe it has been the wrong interpretation by people who translated the original epic throughout the age. Everything in Valmiki Ramayan may not be 100% true. It was written way back in the treta yuga, and throughout all those years since, it has been passed through many hands. Many stories in today's "Valmiki Ramayan" are highly debatable, and if scholars can debate such incidents as Sita's agni pariksha and second exile, as well as Rama killing the Shudra Shambuka, then even this issue of Vishnu "leaving" Vaikunta can be debatable.

The Upanishads' praise of Lord Vishnu explicitly say that he never left Vaikunta. He dwells everywhere in the universe and Vaikunta was only one of the many resting places of the Lord. Occasionally his ansh left for Earth to destroy evil and preserve good, but to think of God leaving one place in the universe for another is totally inaccurate IMO.

Personally, I do not believe Vishnu ever left Vaikunta. To say he "left" and "came" somewhere is to make him human. What then makes God different from humans? He is everywhere in this universe at all times. Neither does he leave, nor does he come.

TV shows and movies can show what they want, and audience can either accept it or disagree, but it is definitely not inaccurate if Vishnu is shown in Vaikunta during Ram avatar and Krishna avatar. It may be unacceptable to some people on the basis of one purana or another, but many other scriptures paint a very different picture of Lord Vishnu who is formless and all pervading. Such a Vishnu can never leave or come anywhere. He is God, not human.