Dear Joel, 

Of itself something like smoking would not be an impediment for following the priesthood, there is certainly a huge difference between it and an addiction to drugs or drink.  

However, it is right to ask if there might be fundamental flaw in a person who has this or some other light addiction, something of which the addiction is only a symptom, and something that would make it imprudent for that person to take on the commitments of a religious or priestly vocation. It will not be so in every case, but in some it might. Some people are just a bundle of nerves, and there seems to be no other way for them to handle them than some outlet like smoking. Sometimes smoking is an escape from an underlying problem or unresolved conflict. At times it can be a sort of subconscious will to self-destruction. I won't get into all of that and other possible causes because I am not qualified.  

Let me just tell you something I have learnt from experience: quite often a teenager smokes out of the natural drive for independence that funnily enough drives him to imitate his friends and become a fashion fiend. He finds plenty of support in acquiring the new, difficult habit (it doesn't come as easily as acquiring a liking for chocolate, it's expensive, and has no practical advantages), but once the addiction takes root, he doesn't find the same support to break loose of it.  

Many are trapped in it despite their desire to quit, like many suffer from overweight. What they often lack is real purpose to break free. But once they have a purpose (a vocation for some, or the preference of the girl they really think they might want to marry, or the condition for a job they really want, or a health scare) they have been able, and often cold turkey. Positive support and doing things that will take their mind off it often helps them on their way. 

I think I went off on a tangent, but I hope it helps. If smoking is not a sign of a major unresolved issue in a person's life he should be hopeful, all he needs is a reason, and then he has to bite the bullet.