Van veers to the left and vibrates

You are more insightful than a lot of owners, but let me clarify a couple of things. You mentioned a vibration from the engine. That, given the history, is due to the new spark plug, or the wire, as you suspect, or an adjacent one got knocked off. The spark plugs do come pre-gapped, but that pertains to certain applications that might not be correct for your engine. Also, it could have been dropped. Regardless, it's customary to check them before installing them. GM engines call for some pretty large gaps, and they have the ignition systems capable of firing spark plugs with those gaps. The correct gap is listed on the emissions sticker under the hood. The place to start is by rechecking the work in that area.

The round thing you mentioned is the cross member. That's a structural part of the frame and is the proper place to raise the front end with a floor jack. It's only natural for the van to lean when the jack isn't centered. That in itself is not the cause of any problem. What DID happen though is something changed the alignment.

You mentioned a vibration while driving. That is never an alignment issue. A vibration is due to a bent wheel, a broken belt in a tire, some other uncommon and elusive causes, but most commonly from a wheel out-of-balance. An alignment will not solve the vibration. The clues are you will never feel a vibration due to tire balance at low speeds. You need to be at highway speed for that. On the other hand, you WILL feel a broken tire belt at low speeds. It will make a thumping in the vehicle at higher speeds, but when driving through a parking lot, you'll see the steering wheel oscillate left and right once per wheel revolution, and / or you'll feel the whole van wobble left and right.

Misalignment will cause three main symptoms. Tire wear takes a long time to show up, as in thousands of miles for minor problems, or dozens of miles for causes that are impossible to ignore. The other two you will see right away. Those are a pull to one side like you have now, and an off-center steering wheel. When either of those things change suddenly, something happened that changed the alignment significantly. There are some cars that will have bent sheet metal just from jacking them up in the wrong places, but your van uses shims that are bolted into place. It's not unheard of for those shims to fall out, but you will have the additional symptom of the alignment changing while you're driving. That is extremely miserable and makes the vehicle hard to control.

It is also possible to hook the steering linkage with the jack and bend part of it. That changes the direction one wheel is steering, but that's all. You have to counter that by turning the steering wheel. That's the off-center steering wheel, but that alone doesn't cause a pull to one side. You've already differentiated a pull from an off-center steering wheel. That's confusing to a lot of people.

The steering and suspension systems should be inspected at a tire and alignment shop. They will determine why the alignment changed, and they can handle the vibration.