Unless you use B6 all the time, and thereby possibly work up physical tolerance towards it, I don't think stopping its use will have detrimental effects beyond simply loosing the benefits. Actually, if it helps make you lucid more often, it will lead to you getting better acquainted with the dream world, which ought to be of lasting benefit (realising that you are dreaming should become easier, the more intimately you know the dream world).

By the way, I have found DMAE to be the biggest booster of dream vividness of all supplements (and I have tried more than I will publicly admit to ).