The question is: What martial arts are similar to Wing Chun?
The question does seem like it is open to interpretation. It depends on which aspects of Wing Chun you're looking at.
But without going into every permutation of different attributes, I want to cut to the chase and mention one martial art that comes to my mind the most. That would be Southern Praying Mantis kung-fu.
Both Southern Praying Mantis (SPM) and Wing Chun have an emphasis on keeping the number of forms to a minimum (especially in the Kwong-Sai Jook Lum branch of SPM). And their forms are designed to teach fundamental concepts rather than just a collection of self-defense techniques.
Both styles incorporate chi-sao (sticking hands) practice. Both incorporate wooden man dummy training. Both are "center-line" dominated styles. Both put two arms forward instead of standing with one arm ahead of the other. Both are close-range systems. Both emphasize hand techniques over leg techniques. Both are hard and soft styles (SPM is a bit "harder" than Wing Chun in my opinion).
I once visited a Wing Chun school in Houston. I had just gotten done visiting Henry Poo Yee's SPM school, and I mentioned it to the Wing Chun sifu. He told me that he had great respect for SPM and thought of it as a kind of "sister style", as he put it. He explained that the two styles shared much of the same philosophy and tactics.
It takes a lot longer to learn SPM than Wing Chun, though. Some schools of SPM have you learn one form and repeat it over and over again for anywhere from 1 to 4 years before you're allowed to learn anything else. It also incorporates internal mechanics, which Wing Chun styles do not do. Internal mechanics requires a lot more training and refinement.
In SPM, there's also a reliance on the phoenix-eye fist (a single knuckle projecting from a closed fist), which is used to attack pressure points primarily. Wing Chun has no emphasis on pressure point striking. So there is more detail in SPM about what to strike and how, which requires a lot more time to learn.
So my advice is to look into Southern Praying Mantis. In my mind, nothing else comes as close to Wing Chun. But keep in mind that it will take longer to learn.
By the way, don't confuse Southern and Northern praying mantis styles. The southern styles of mantis are very different from northern styles. Southern Praying Mantis doesn't even make the "mantis finger" thing that you see in Northern Praying Mantis styles. It's completely different.
There is one style I think you might also want to check out: Taiji (also spelled Tai-Chi). You know, that slow, graceful style you see people practicing in the park.
Both Taiji and Wing Chun claim to be based on Shaolin kung-fu, and specifically Snake and Crane styles. Taiji incorporates push-hands practice, which is similar in a way to Wing Chun's chi-sao practice. Both are attempting to develop snake style's "listening" soft skill, where you use the connection with your arms to your opponent's arms to sense what your opponent is about to do. It can cut down on reaction time greatly. And while you're at it, you're yielding to force instead of using force against it.
But Taiji is an internal art, which requires decades to learn and master. It does have its own fundamental techniques and principles, just like Wing Chun has. But it doesn't look anything like Wing Chun. If anything, you might say that Wing Chun is taking one small aspect of Taiji and utilizing it for everything. Taiji has a lot more stuff to learn.
One other style that comes to mind that is somewhat similar to Wing Chun is Kali / escrima. The Hubud exercises they do look a bit like chi-sao, but with more percussion and with less sensitivity.
Kali also involves a simple, logical set of hand techniques based on angles and directions. Most of its leg movement is based on the triangle shape. The combination should seem very familiar to Wing Chun people who have their own logical arrangement of hand techniques and stepping.
Now, if the aspect of Wing Chun that you like the most is how quickly it can be learned, then I'm afraid both SPM and Taiji are not for you, if you haven't already guessed that by now.
But Kali actually does qualify. It can be learned in a very short amount of time. Within about a year, you can pick up the basics of the system and be able to defend yourself reasonably well using it.
What's more, what you learn with sticks (escrima) can be applied to empty-handed, knife, and machete fighting with almost no changes. So it's a very time-efficient style to learn.
The application of Kali for machete, stick, and knife fighting is reminiscent of the way Wing Chun approaches butterfly swords and staff fighting. In Wing Chun, butterfly swords and staff are really just an extension of what you already do in empty hand training. The same is true of Kali and the way it approaches the machete, stick, and knife.
The biggest difference between Kali and Wing Chun is the fighting range, however. Wing Chun is most comfortable being in the close range. That's where it excels. Kali is more of a mid-range fighting system, because of its emphasis on weapons. In fact, most Kali schools start you off learning with sticks, as opposed to starting off empty handed.
In addition to Kali, I would steer you towards two other martial arts that are time-efficient: Krav Maga and Gracie Jiujitsu.
Both Krav Maga and Gracie Jiujitsu emphasize bringing the students up to speed quickly on a broad range of fundamental techniques and strategies that are meant to keep you alive in a real fight. Training for just one year at those styles should prepare you to defend yourself in real life. That's something that people often aren't able to do even after 10, 15, or 20 years of other styles.
But that's where the similarity with Wing Chun ends. Krav Maga and Gracie Jiujitsu don't look a thing like Wing Chun. At all.
So take your pick: Southern Praying Mantis, because it has a lot of similarities with Wing Chun. Taiji, because it's also a Snake and Crane style. Kali, because of its similar logical approach to attack and defense, as well as its similar approach to rapid learning. Or Krav Maga or Gracie Jiujitsu, because of their emphasis on rapid learning.
Hope that helps.