Thought the Hunter Vanguard spot was dropped since there’s no one to take the spot and the Vanguard aren’t really the Vanguard these days.

The plot has never been dropped. It's always been a weird plotline but it's never been dropped. It was referenced again last season in the second entry in the "Beneath the Endless Night" lore book. Targe scalds Zavala (and Ikora, although she's not there) for doing the work of three people, and tries to encourage him to ask Ana Bray again to take up the position before the two get interrupted by a message from Empress Caiatl. The position itself is probably not going to be resolved anytime soon; at the very least two more seasons; but it's starting to be heavily presented that the one to fill the position will be Crow. Especially now that he has his memories again.

Uldren can not be the Hunter Vanguard as Cayde specifically said in his mission way back in Forsaken: “To the HUNTER who kills me” and Uldren at the time was not a Hunter so he is immmediately disqualified

I'm aware of that. But the Dare has pretty much been left open to the Hunters (in a similar state to how Hunters decided who should take it up when Cayde and Andal made their dare). Crow is however a Hunter, outside of his attire, voice lines during Season of the Hunt indicated he's practising to use Golden Gun. My point was that Crow has his memories, or at least has the perspective of Uldren's actions. This could create some level of guilt or remorse for what Uldren did/was forced to do, and thus Crow might seek to redeem his old life or make right by the actions by taking Cayde's place. It's also a case that he's the only Hunter who's actually working closely with the Vanguard (that we're aware of). Shaw Han is operating out in the Cosmodrome and Shiro-4 is currently doing something else since his previous assignment to keep tabs on Lakshmi-2 is no longer happening. Most of the other named Hunters haven't been mentioned or are one-off appearances in the form of minor characters.

Most Hunters haven’t taken up the Dare because Hunters belong in the Wild and they all know that. If Uldren was a good Hunter he’d stay far away from the City. If he ever were to take the position there would be riots as far as the eye can see

Firstly, he's not Uldren, he's Crow. Uldren is long gone. Secondly, A lot of those close to Cayde or those in leadership positions have seen Crow and moved to forgive him for UIdren's actions. Zavala, Ikora, Saladin, Us and our ghost and even Petra. A large majority of Guardians understand that a Guardian isn't considered the same person they were in their previous life. Uldren and Crow, like a large majority of other Guardians, have similar aspects, but that's where it ends. Uldren stopped being Uldren when the Forsaken Campaign ended. Crow is who he is, and I'm sure a majority of Guardians, especially Hunters would understand that. And the "riots" likely wouldn't come from many. I doubt lightless people, or the Eliksni would be bothered by it, and those -blam!- the Vanguard will likely either side with putting Crow in power or just not be bothered. Heck I'd imagine a large majority of Hunters would be in favour of it; even if it was just because someone is filling the position.