I played the Viking DLC first, and loved all the options with the sandbox mode, the character creation with the background lore, and the towns full of NPCs that you can interact with and people who talk to you differently depending on what you are. Then I tried out the vanilla Warband, and it doesn't have a sandbox mode, way less starting options, and the world feels lifeless. I was kinda surprised. I love the viking dlc though. Feels more like a sequel to Warband instead of a dlc since it expands and adds so much new stuff.

Most of the Mount & Blade titles -- especially the original and Warband -- are little more than sandboxes. Mount & Blade has no overarching story at all, and Warband, IIRC, just has a short initial quest chain that serves as a kind of tutorial, and can be completely ignored if you wish. (Not that it'll usually take very long to blow through if you choose to do it.) It would be truer to say that there is no story mode in these two games. (I don't get the impression there's a tremendous amount of plot or much more of a main quest line in With Fire and Sword either, but to be honest, I've seen very little of that game. Bannerlord, on the other hand, seems to be going for something a bit closer to what Viking Conquest sounds like, with a somewhat more involved overarching quest chain.)
Fair enough with the "lifeless world" criticism -- at least compared to a lot of proper plot- or quest-focused RPGs, which this franchise isn't, overall. The early games are almost entirely about battles and chasing down/running from enemy parties on the "overworld" map.

WRT Viking Conquest: It should be noted that it's basically a remake of a mod for Warband (maybe originally for the base game? I don't remember anymore), made by at least some of the people who made that mod and released by TaleWorlds. So if it feels substantially different and fuller-featured, that's because it was not only made by a different team, but based on something that was already trying to seriously expand and alter the game.

Strange, I love M&B: Warband but I just couldn't get into Viking Conquest.
Maybe it's because you start in what is the current day Netherlands, which looks like a completely barren desert, while in reality in those days it would either be a marsh, heavily forrested or inland seas.

I must say that I will probably try it again but I at the moment I hold it in the same dislike as Fire & Sword which I also didn't like because of the same reasons. If you go historical then it must also be believable as a historical setting, Calradia was just a fantasy setting which made it less offensive to me I guess.

The base game itself offers so much replayability that I am yet to pass on the With Fire And Sword, and later onto the Viking Conquest. What I have noticed in Viking Conquest is that the facial features remain almost the same across all the characters. But it is so good to see all the additional content like children in towns, groups of lords joining king to form a massive army, which is visually represented on the world map, religious influence, guilds, the number of factions, the size of the world map. Just wow.