Yes, the plan was to build the pipeline to Hungary and to get Russian oil through Hungary and Ukraine, if the USA would allow that. The problem is that it is planned to be finished in 2027.
There were several rumors about that, but so far nothing is certain because RF would lose political influence in RS if it was economically removed from RS.
There were never plans for the oil pipeline to Hungary. There were talks about that. Talks mean nothing. Any pipeline which doesn't go through Croatia (Adriatic pipeline) was out of the question in the last 10 years or so, because it would mean a loss of big money for serbo - croatian organized crime which controls both countries for the West, who also doesn't want that. The pipeline Southern Stream was expected to do that job, but it was stopped by Bulgarians. Infrastructure already built still standing.
There were no talks between Serbia and Russia. There is no Serbia in that. Only Vucic (and the gang) and Russia. Vucic is not interested in talks about NIS with Russia, Russia doesnt believe Vucic any more.
Also, important, Vucic signed last year the deal with the US of "strategic energy partnership".
Not to forget, Hungary is importing Russian oil without problems. The part of oil and oil products coming from NIS to Serbia is about 30% of the Serbian market. NIS covers 30% of the market. The rest is from imports. Why is that important? Because, Serbia "will not get into a halt" if NIS stops, as Vucic is saying. There is no need to "scrape the barrels", "take military reserves", "stop the bakery to bake bread", or to "double the prices of gas and diesel", as Vucic melodramatically announces EVERY DAY on TV. Moreover, Hungary said that their company MOL will replace any loss of percentage of oil to NIS if Serbia solves the problem with NIS. There is no need for expensive US, Iraqi oil which comes through Adriatic oil pipeline at all, MOL would provide cheaper Russian oil, all the way until Serbia and Hungary build connection to Russian Druzhba pipeline.
But then, what is the problem?
Its actually quite simple. NIS is owned by Serbia and Russia. At some point USA said: "no Russian STATE OWNERSHIP" in any oil companies in Europe, no "NOT RUSSIAN OIL". Oil and gas are permitted. It flows. In the agreement between Serbia and Russia, when the NIS was partly sold out to Russia, there is a clause which forbids Serbia to nationalize (take over by the government's decree) Russian part. If Serbia nationalize Russian part of NIS, Russia have the right for international arbitrage and settlement. That means that NIS would be shut down (NO MONEY OF IT - bad). There is solution where Russia sell its part of NIS to Serbia or some third party. But Serbia as a state (country, in american English) never offered that officially, Serbian part of the NIS never offered officially to buy out Russian part. There were rumors that some third parties offered to buy it (western hedge funds, serbo - croatian organized crime hedge funds, UAE, even Hungarian MOL, which would be real offer for buying out and the best for Serbia), but Russia remains silent. All that time Vucic is saying that he will solve that with the US. Never that he will solve that with Russia, which IS the actual address for solving the problem.
So, still no VISIBLE problem in all that, BUT, behind the curtains:
(I am aware that I am boring with this, and this is hard to grasp that some country in Europe can be so corrupted and politically devastated, but we are forgetting Ukraine, much bigger and stronger than Serbia. So its possible. Bear with me, please).
Vucic is a psychopath and a criminal. Plain and simple. This is important because that is the starting point of the problem. He is not doing anything for the people of Serbia, or country of Serbia. He is doing only for himself. Vucic is not Serbia. Also, his days in the power are getting shorter. He had couple of deals with the US (NIS is not the only one), of course with unofficial, or semi-official groups, which would buy him some time at power, granted bu the US, insane sum of money, and possibly protection if he have to flee from Serbia. The US gets money, firmer control over Serbia and thus the Balkans, and international law precedent for the future.
The plan is to take over Russian part of NIS, or to buy it himself through some hedge fund, and than after squeezing and stealing some more money for himself of it, sell it to some US hedge fund, of course for a hefty provision for himself (hundreds of millions or billion level, he's way past just mere millions) and possible shelter somewhere under the US dominance. He cant nationalize it, because that would mean international arbitrage and no money, possibly even not reselling. He cant buy the Russian part, because Serbia have no money, and that would mean less money for him, possibly less money when reselling. He doesn't want Russia to sell their part to some third party (MOL for example) because he wouldn't be able to suck out more money of it and than resell it.
He was HOPING THAT RUSSIA WILL GIVE THEIR PART FREE TO SERBIA, so that "SERBIAN PEOPLE DOES NOT SUFFER".
At the same time, Vucic is in campaign of westernisation, NATO-isation (for the lack of better term...) of Serbia and blatant defamation of Russia. So we have in his TV appearances (every day) and his media (and all are his) and thousand of his bots on social media stuff like this: "Russia is a false friend, they are permitting the US to choke us", "Ukrainians are wining the war and busting Russians", "we will not have petrol because of russians", "Russia dont want to sell it to us"and so on.
Russia is silent on all that. As a person living in Serbia, I can only hope that they are aware that Serbian people are actually Vucic'c hostage. If they are not, and do something that benefits Vucic, we are doomed. (the latest from Putin, Zakharova and Peskov, and unofficially from Dugin tells that they are aware. There is hope.)