When I view the community through the lemmy instance I'm using, I'm seeing no posts there. I guess this is a problem related to vlemmy but just putting it out here if someone else is having the same problem and is confused https://vlemmy.net/c/ufos@lemmit.online
edit: now it shows the reddit threads there, so probably as I understand vlemmy hadn't seen them yet (as no one in vlemmy had been accessing the community yet) and it took it some hours(?) to sync the stuff from another instance.
There are two things slowing down the migration. One is the "network effect" https://en.wikipedia.org/wiki/Network_effect (which is really strong in reddit) and other is the confusion and bugginess of Lemmy right now as lemmy is still in its infancy. Decentralization makes this a bit confusing to use and it doesn't help that some posts etc are not showing up due to bugs or lack of features so you are not sure if the problem is you or lemmy but regardless of all that, federation is better than the reddit empire so we're on the right track