So far people have surmised that either A) The Vision's status as an Android exploits a loophole in the rules regarding the wielding of Mjolnir, as set by Odin, because The Vision, as an artificial intelligence encased in a android body, isn't a person. Or, B) that The Vision, being newly created and therefore not yet compromised or corrupted in any way, and with The Vision having been created via the combination of Jarvis' benevolent artificial intelligence, downloaded into an android neural network designed by Ultron and powered by the Mind Stone, and is therefore "worthy" of wielding Mjolnir.

But I'd like to point out a couple of flaws in those theories...

As for the first theory, that The Vision isn't a person and can therefore lift Mjolnir in the same way as an elevator or heli-carried or truck would, it should be pointed out that The Vision didn't just lift the hammer - he threw it, caught it, hit Ultron with it, etc. It should also be noted that Odin, and Asgardians in general, aren't at all unfamiliar with androids - and I don't believe The Destroyer can lift Mjolnir.

As for the second theory, that Mjolnir considers The Vision to be worthy of wielding it (and ruling Asgard too - let's not forget that!), I would point out that there was no "power-up" when The Vision first picked up the hammer. He didn't suddenly gain armored sleeves, a cape, a, ornate helmet, etc., such as what happened when Thor became "worthy" in the first Thor film, or such as what happened when Jane Foster became the new Thor. And, while The Vision did wield Mjolnir in the battle with Ultron, he wielded it in the hitting/throwing sense of the word, but didn't appear to have mastery over thunder and lightning.

If I may offer an alternative theory, it could be that the source of The Vision's power, the Mind Stone, is one of the "Infinity Stones", and is therefore am ancient and mystical object of immense power.... Power that is far older and stronger than any spell/curse that Odin could possibly cast on Mjolnir (or any other object). In short, the Mind Stone's power was unaffected by Odin's rules or even the powers granted to the person/people worthy of wielding the hammer. Basically, that becoming the new Thor would be something of a demotion for The Vision.

Lastly, as a side note, to address the comments of those people who mentioned various instances when Captain America, Hulk and others wielded Thor's hammer, I would point out that each such instance happened in the comic books during a period of time in which Odin had taken Mjolnir away from Thor as punishment for Thor's defiance in not leaving Earth and returning to Asgard, as Odin had ordered. Following Thor's being stripped of Mjolnir, Tony Stark created a new hammer for Thor (actually, a hammer on one side and an axe on the other). The new hammer was as close as Stark could get to the actual Mjolnir as earthly physics and cutting-edge earthly technology would allow. But the new hammer did have it's limits and the whole "worthiness" thing was a one of the major ones, as pretty much anyone with enhanced strength could lift it.

So far people have surmised that either...

A) The Vision's status as an android exploits a loophole in the rules regarding the wielding of Mjolnir, as set by Odin, because The Vision, as an artificial intelligence encased in a android body, isn't actually a person, but a machine.

Or, 

B) that The Vision, being newly created and therefore not yet physically, mentally, spiritually, emotionally or morally compromised/corrupted in any way, and with him having been created by downloading Jarvis' benevolent artificial intelligence into an android brain/neural network designed by Ultron and powered by the Mind Stone, and that The Vision is therefore "worthy" of wielding Mjolnir.

I'd like to point out a couple of flaws in those theories...

As for the first theory, that The Vision isn't a person and can therefore lift Mjolnir in the same way as any elevator or heli-carrier or Quin-Jet would, it should be pointed out that The Vision didn't just lift the hammer - he threw it, caught it, flipped it, hit Ultron with it, etc. He, in effect, "wielded it". It should also be noted that Odin wouldn't be at all unfamiliar with androids or artificial intelligence - but I don't believe The Destroyer can lift Mjolnir either.

As for the second theory, that Mjolnir considers The Vision to be worthy of wielding it (and ruling Asgard too - let's not forget that!), I would point out that there was no "power-up" when The Vision first picked up the hammer. He didn't suddenly gain armored sleeves, a cape, an ornate helmet, etc., such as what happened when Thor became "worthy" in the first Thor film, or such as what happened when Jane Foster became the new Thor. And, while The Vision did wield Mjolnir in the battle with Ultron, he wielded it in the hitting/throwing sense of the word, but he didn't appear to have mastery over thunder and lightning like Thor does.

If I may offer an alternative theory, it would be that the source of The Vision's power, the Mind Stone, is one of the five "Infinity Stones", and is therefore an ancient and mystical object of immense power.... Power that is far older and stronger than any spell/curse that Odin could possibly cast on Mjolnir (or any other object). In short, the Mind Stone's power was unaffected by Odin's rules or even the powers granted to the person/people worthy of wielding the hammer. The Vision, as the embodiment of the Mind Stone is simply too full of ancient power to be subject to mystical limits placed on objects by Odin, who, to The Vision, would be an inferior being. Basically, that becoming the new Thor would be something of a demotion for The Vision.

Lastly, as a side note, to address the comments of those people who mentioned various instances when Captain America, Hulk and others wielded Thor's hammer, I would point out that each such instance happened in the comic books during a period of time in which Odin had taken Mjolnir away from Thor as punishment for Thor's defiance in not leaving Earth and returning to Asgard, as Odin had ordered. Following Thor's being stripped of Mjolnir, Tony Stark created a new hammer for Thor (actually, a hammer on one side and an axe on the other). The new hammer was as close as Stark could get to the actual Mjolnir as earthly physics and cutting-edge earthly technology would allow. But the new hammer did have it's limitations, and the whole "worthiness" thing was one of the major limits, as pretty much anyone with enhanced strength could lift it.