I saw a clip of Asuna comparing the first shot spread of the phantom and the vandal, saying that the vandal's difference of 0.05 fss makes your crosshair placement irrelevant. "The vandal misses, it's not even accurate on the first bullet. That's why no one uses this gun. You can aim at the head and you will not kill people, because your crosshair placement is irrelevant with that gun." (The clip in question: https://youtu.be/oeZDbL6tMIg)

So i did some testing, comparing the first shot accuracy of both guns at 5, 10, 20, 30 and 50 meters. Here are the results:

5 meters: https://imgur.com/a/gafZtNu
At 5 meters the difference is negligible. Both guns will one shot head shot at this range.

10 meters: https://imgur.com/a/2iLXcuJ
At 10 meters the difference is still negligible. Both guns still one shot head shot.

20 meters: https://imgur.com/a/0md2YIg
At 20 meters we start to see a very slight difference. While still miniscule, you might miss with the vandal if you are aiming at the very edge of the head and get unlucky. Of course the same thing may happen with the phantom, but at much, much lower rates. Keep in mind however that the phantom will not kill in one headshot at this range, so the vandal rewards good accuracy with a lower time to kill, whereas the phantom allows you to be a bit more inaccurate with the drawback of a higher time to kill.

30 meters: https://imgur.com/a/oIjD5KX
At 30 meters the difference is a bit more apparent. Same thing as 20 meters, you might miss a shot with the vandal if you're not aiming right between the eyes.

50 meters: https://imgur.com/a/WUybjfW
At this range the phantom is the clear winner, though you usually won't fight this far apart, in my opinion the consistency of the phantom beats the one shot of the vandal. It's more of a "will the vandal user get lucky enough" scenario if both players fire at the same time.

Conclusion: At standard fighting ranges the difference is small enough to pretty much not matter, it really just depends on what you prefer to play with.