I'm an ex-cigarette user who made the transition to e-cigs a while ago. Like the other forum member has said, the benefits of vaping far out weigh the traditional way of having a 'bifter'. I was smoking Marlboro lights and they are now over £13 a pack at the local filling station near me! I also prefer the taste of the vape and others around you notice it because cigarette smokers usually smell of nicotine (despite the fact you might be the most hygienic person ever) and I certainly don't miss that nauseating feeling you get after you've had a cigarette.
To answer your question - cigarette's and e-cigs have absolutely no impact on the medicals. Clearly the doctors/nurses are looking for evidence of drugs that fall under tbe category of controlled substances that can stay in your system for months after. For example, cocaine can still be detected in hair follicles three months later.
Alcohol is probably the biggest stumbling block for a lot of people. The powers that be in the railway industry don't want their employees to be tee total but there has to be a degree of self discipline if you enjoy a drink. Again, if you are a heavy drinker or someone hovering around the alcoholic zone then your liver functions in the medical test would show evidence of alcohol abuse. Like me, if you just enjoy drinks at the weekend or the odd one on a week night, my advice would be to abstain from any alcohol for at least a week prior to a medical.
C J