As we know by now, the Vita is pretty much dead in the west. (It still has decent sales in Japan though)

Personally, I don't think it deserved to fail. Not at all. I own both a Vita and a 3DS, and I actually have more games for my Vita and play on it more than my 3DS. I would recommend both systems if you can afford it. After all, even buying both costs less than just 1 PS4 or Xbox One.

But, it is what it is, and its sales outside Japan are dismal, selling behind even the Wii in some weeks! So, in your opinion, what was the main factor resulting in the Vita's failure?

All of these factors in the poll contributed, but what do you think was the biggest reason? Personally, I think it was a lack of advertising. I've never seen an ad for the thing. And many people don't know it exists. For example, when I'm playing on my 3DS, some people says "Oh, you have a DS?" whereas when people see me playing on Vita, they say "Hey, what's that?"

So, what happened? Could Sony have done anything to make it more succesful? Or was it doomed to begin with?

If I had to order what were the 3 biggest reasons for Vita's failure it would be
1. No big, must have titles
2. Smartphones/3DS
3. Too Expensive

Also as you can see from latest charts, Vita is even declining in JP, it did less than 10k on the latest charts :/

No big game release is the reason why the Vita ended up this way ...

Smartphones happened. It never had a chance.

Sony gave up too easily after the first holiday and the library being filled with games that mostly target the niche market instead of something broad. As well as super over priced memory cards. Oh and massive amounts of ports as well as watered down multiplats (graphically and performance wise)

1. Poor naming. It broke the PS naming convention and confused consumers. Most people don't know what a Vita even is. The PSP2 would have sold phenomenally better.

2. Too little advertising. The Vita has the most remarkable lack of advertising I've seen. Given the unwise, artsy name, it would have needed a lot of ads to raise public awareness. Xbox One is a stupid name, but MS's ungodly ad budget makes up for its inherent awareness deficit.

3. The wrong kind of advertising. PSP ads helped to sell an awesome gaming machine by also showing off its then-remarkable multimedia features. Sony tried the same with the Vita, but smartphones had made this emphasis a null point. Thus, it didn't look like something people "needed."

4. Handheld decline. The Vita is doing horribly compared to the PSP, but the 3DS is doing horribly compared to the DS. This gen has seen a massive handheld decline, most likely attributable to smartphones and tablets. The market itself is much smaller, and this actually accounts for a surprising amount of the PSP-Vita decline.

5. Digital focus. The Vita has/had too strong a digital push, resulting in limited shelf space and lack of awareness of its games. The expensive memory cards only make it worse.

6. Inconsistent messaging. Once the Vita had all of the problems, Sony did not develop a coherent strategy to rebrand the Vita the way Nintendo did with the initially failing 3DS. Indie machine? Niche Japanese gaming? Remote Play hub? Multimedia device? Console games on the go? The truth is that it is an amazing nachine -- all of these things and more. Alas, this creates a confusing identity for the system and results in hyperbole by internet halfwits claiming that the system is "abandoned," which is clearly untrue to a hilarious degree. However, that affects other uninformed people and creates a common myth that works against the system while it continues to receive and abundance of titles and features.

EDIT:

To address your last questions, I'll offer this synopsis: The Vita was doomed to an extent from the beginning due to handheld decline and poor market positioning in the rise of smartphones. However, it could have done better than it has with a smarter name, more advertising, and a tighter PR message to sell people on what the device is supposed to be. I find the Vita incredibly easy to recommend to people and always find them very happy with the purchase. Sony could have done the same. 

Mind, the Vita continues to receive phenomenal games support and has the most "core" audience of any system, meaning that Vita owners buy more games per person than what we see on any other platform. Furthermore, it continues to sell steadily and even sell out of its low production capacity at times. The Vita is not abandoned and is not a failure; it is a vital component of Sony's goal of creating a vibrant PlayStation ecosystem. However, it is also not what it could be.

Untrue. The system literally LAUNCHED with a fantastic Uncharted title and continued to receive major AAA initiatives from old IPs (Killzone, LittleBigPlanet, Assassin's Creed, Persona, CoD (albeit not a very good game), etc.) and new IPs (Tearaway, Gravity Rush, Freedom Wars, Soul Sacrifice) in addition to unprecedented indie support, robust integration with the PS3 and even more so with the PS4. Simultaneously, it has unprecedented indie support and intense support from the niche Japanese games sector.

Games are not and never were the Vita's struggle. In fact, Vita owners continue to buy more games per system than what is done on any other system.

The Vita's library has become more niche over time, but this is in response to the sales, not the cause of the sales.

I'm hearing smartphones cited a lot as reasons. The issue I have with this theory is that smartphones don't seem to have had a major impact on the 3DS.

Yes, the 3DS is selling less than its predecessor, but I think that's more to do with other reasons, such as confusing naming, lack of innovation, and being expensive (compared to the DS at launch).

1-Lack of must have titles
-Library flooded with poor ports and multiplats, and indies
2-hard to hack (compared to PSP)
-Everyone I knew who had a PSP had it hacked
3-Less marketing (I SUPPOSE)
-Vita marketing is maybe 20% less than what the PSP had, in my country.

Monster Hunter.