I don't think you need to worry about it unduly; all your hand is doing is falling into a natural position. Mine does exactly the same; I don't know how common that is -never really thought about it- but I'd think very common indeed among players in traditional as opposed to orchestral styles. If you want to play the "classical" repertoire, or play easily in high positions, you'll perhaps have to develop that unnatural hand position in which the hand doesn't touch the neck in that way, but it can put a lot of strain on the wrist. There is more than one way of getting vibrato, and I manage well enough with the same sort of hold you're using; in any case, you usually don't need much vibrato for traditional music. Over-use of it (as also in singing) identifies the classically-trained musician who just can't shake the habit, so to speak.