Worth Playing Vanilla or Just Skip Straight to the Xpac?

Why don't you start with DL, then try DA, and see how you feel about both versions?

They both have their own merits and downfalls, after all.

Playing without the expansion would make it easier to learn the ropes, because it gets a bit more complicated with all the new stuff. However, the expansion turns it into a much more fun (and richer) game.

Is the expansion's campaign significantly harder that DL? Or does the DL campaign provide a more gradual introduction to the game concepts?

I just played the "quick start guide" in DA and didn't have too much trouble, at least I haven't thus far, hostilities are yet to commence! Anyway, I'm currently leaning towards sticking with DA unless DL is able to better "teach" the game.

In my opinion, neither is better for teaching the game, but there are some big differences in them. The big one is ship combat. Individual weapon mounts target INDEPENDENTLY in DA. This means you can have a single ship attacking (and possibly killing) multiple ships in a single combat round. In DL you can be successful swarming a capital ship with tiny or small fighters, in DA this usually just results in dead fighters.   

I tend to agree with Evil Muppet that the core version should be a bit easier to learn, simply because there are fewer "parts."

As to which will be most fun for you after you get the basics down, that's a matter of taste.

Some old hands appear to dislike some new stuff in DA, e.g. the extreme environments or the radically different spying system. Others of us are not likely to play another DL game because we're too fond of something like the independent weapon firing that Deathgerbil mentions. I'm a classic mixed case: I'm staunch critic of the lack of passive intelligence gathering in DA, but the environment stuff, combat changes, and new treaties have me hooked.

I feel that DA is superior to DL, but I still recommand playing thru the DL Campaign for the story. Then play the DA campaing to complete said story.

You will have a better learning experience with the original DL. It's easier to manage your economy (tbh, you barely have an economy at all in the expansion) and you can get a handle on the whole colonization and buildup portion of the game more quickly without worrying about the extreme planets, which the AI always seems to grab first.

Also, diplomacy and tech trading are much kinder and more useful to the player in the original. I would suggest at the very least playing DL until you feel comfortable with the mechanics and can win the game at Tough level or better, then moving on to the expansion. And definitely play a few normal games, not just the campaign, to get the full tech tree and play against different opponents.