Although I am not a doctor, I am a medic with six years vaping experience along with 15 years in the field emergency medical experience, and can only speak from that point of view/back ground on the subject matter proposed. The short answer you are looking for is that yes, the doctor(s) is/are correct in stating that nicotine can be really bad for the heart; especially if a person has a known or unknown compromised cardiovascular system. The amount to which the nicotine is detrimental to the cardiovascular system is directly dependant on the individual and specific to his/her current physical/mental health and past physical/mental history. Therefore, there is no one answer to the question of whether nicotine will have permanent chronic or acute detrimental effects upon an individual, but it can be generalized that the introduction of ANY substance (including water in some cases) can have a negative effect on the heart. Whew! Now that I got that mouth full out of the way, let’s start talking where I can understand myself..... Nicotine is a stimulant, much like caffeine, and increases the processes of the of the body; (and not getting tooooo specific here...) including the heart rate, blood pressure, metabolic rate, electro-chemical reactions/processes, and other bodily functions. That's why we feel able to react faster, think better, and generally function better when we vape. It's a "pick-me-up"! Remember, nicotine is a drug. We generally don't use drugs because they don't work. We use drugs because they work, PERIOD, and in such a way that we think benefits us in some form or faction. Whether recreational or pharmaceutical, we use drugs for a desired response/reaction. However, since nicotine speeds up the heart rate and increases the blood pressure, it puts extra added stress on the total cardiovascular system (heart and pluming). Putting stress on an otherwise healthy heart and vascular system for short periods of time, doesn't tend to cause permanent damage; think of running or being frightened. However, if you put extra added stress on the cardiovascular system over long periods of time (remember, I've been vaping for 6 years now) and the associated acute conditions of hypertension, elevated heart rate, stress on the vascular walls, etc. can lead to chronic conditioning of your body to try to compensate for the added stresses. Just as any other muscle in the body, if you continuously stress it out, it gets thicker and more fibrous. In the heart, a thickening of the muscle walls leads to smaller heart chambers and a less effective ability of the heart to pump blood throughout the vascular system. This can lead to a condition known as "Congestive Heart Failure" or CHF (a chronic condition). Just like any other mechanical pump, the heart can weaken with age and over taxing. The more stress you put on it, especially over a period of time, the more likely it is to fail. Extra added stress on the vascular system also wears it out, reduces the elasticity, and makes it more brittle. A brittle artery can lead to stroke; whether embolism (blockage) or hemorrhagic (bleeding). (Think of an old worn out garden hose that breaks/leaks when you apply water presure.) Now, each individual reacts differently to the added stress of the stimulant effect of nicotine, and some can tolerate those effects while others may not be able to. Talk with your doctors and try to track your bodily/cardiovascular changes with them over a long period of time. In my personal experience, I know I have hypertension due to my diet, activity level, extreme work stresses/environment, adverse working and sleep schedules, nicotine/caffeine intake, and other contributing factors. However, I am tracking changes and have had a complete cardiovascular work up as a bases to track those changes. There are a couple things I can do (if so inclined) to compensate for the added stress of nicotine use. I can change my diet and reduce my salt/fat/carbohydrate intake, exercise more often with moderate cardio types of activities, start blood pressure medication, or even change life habits/work environment to compensate for the added long term stress that nicotine places on my body. I was a smoker for 27 years without thinking this way or the consequences of smoking in general. Then, like most people, it started catching up with me, and it was either quit or find a replacement. Since all other nicotine replacements failed miserably, and psychoactive therapies have too many side effects, vaping was the only choice I found to reduce the harm my nicotine habit construed.
As far as studies already conducted, here is a great place to start:
http://onvaping.com/the-ultimate-list-of-studies-on-e-cigarettes-and-their-safety/