Nothing external will prevent you from making the shift. All that matters is your
level of consciousness or "vibration" as some like to call it. There is a certain limit that needs to be reached in order to "make it".
Many totally negative people are non-drinkers, non-smokers etc. but they will most likely not make it. Many more evovled people still smoke through old habit and many also don't shy away from a good party once in a while. That is no problem. Excessive use of alchohol/drugs is a sign that you are not of a very high vibration and thus you will most likely not make it.
But never say never. In these times I see many people who were totally "lost" (drugs/alchohol) suddenly go through profound changes. Remember that many have choosen (in pre-incarnation agreement) to "condense" their remaining heavy density karma into one life in order to "make the shift". Such people often go through a very rough period to "burn off" their lower vibrations before they suddenly ascend very quickly up the ladder of consciousness.
True change comes from the inside and radiate out, and not so much from self-control from "the outside in" allthough it is important to have some of that too. Ascension is not really a "doing", but rather something you allow to happen by remaining open and accepting your lessons and "destiny".