Hextech Lockboxes Pattern? :O

Alright, so basically speaking, I have unlocked about like 8-10 hetech boxes so far. Now the thing is that, I have been trying to get my hands on a Vayne skin. Now, Vayne is my most played champion and the highest ranking of em all, and of course my favorite. I kind of expected these boxes to give me skins based on which champ I play a lot. But I have received about 4 champs or wards, which is fine. But of the 6 skins I received, 2 were for poppy, a champ I rarely play. And the rest were for champs I don't even own. Lol. to be kidding me. I thought I would be teed any skin for the champ (I was that desperate) but my efforts were in :(

So is this hextech system completely random or does your favorite champ actually gain some kind of priority?