Tobacco and marijuana are both plants, and many of the same harmful chemicals would be generated when burning any organic plant material. So, smoking marijuana is likely to cause similar effects as smoking a comparable level of cigarettes. As for vaping marijuana, I found a few small studies, which are pasted below. These studies suggest that there may be benefits to vaping marijuana compared to smoking it, but these are pilot studies that need to be validated. Vaping marijuana may still be causing some level of irritation to the lungs, and this concern needs to be weighed against the medical benefit.


Int J Drug Policy. 2010 Nov;21(6):511-3.
Pulmonary function in cannabis users: Support for a clinical trial of the vaporizer.
Van Dam NT, Earleywine M.
Abstract
BACKGROUND:
Debates about cannabis policy often mention respiratory symptoms as a negative consequence of use. The cannabis vaporizer, a machine that heats the plant to release cannabinoids in a mist without smoke and other respiratory irritants, appears to have the potential to minimize respiratory complaints.
METHODS:
Twenty frequent cannabis users (uninterested in treatment) reporting at least two respiratory symptoms completed subjective ratings of respiratory symptoms and spirometry measures prior to and following 1 month's use of a cannabis vaporizer in a pre/post-design. Outcome measures included self-reported severity of nine respiratory symptoms as well as spirometry measures, including the maximum amount of air exhaled in 1s (forced expiratory volume; FEV1) and maximum total lung volume (forced vital capacity; FVC).
RESULTS:
The 12 participants who did not develop a respiratory illness during the trial significantly improved respiratory symptoms (t(11)=6.22, p=0.000065, d=3.75) and FVC, t(11)=2.90, p=0.007, d=1.75. FEV1 improved but not significantly t(11)=1.77, p=0.053, d=1.07.
CONCLUSIONS:
These preliminary data reveal meaningful improvements in respiratory function, suggesting that a randomized clinical trial of the cannabis vaporizer is warranted. The vaporizer has potential for the administration of medical cannabis and as a harm reduction technique.

Harm Reduct J. 2007 Apr 16;4:11.
Decreased respiratory symptoms in cannabis users who vaporize.
Earleywine M, Barnwell SS.
Abstract
Cannabis smoking can create respiratory problems. Vaporizers heat cannabis to release active cannabinoids, but remain cool enough to avoid the smoke and toxins associated with combustion. Vaporized cannabisshould create fewer respiratory symptoms than smoked cannabis. We examined self-reported respiratory symptoms in participants who ranged in cigarette and cannabis use. Data from a large Internet sample revealed that the use of a vaporizer predicted fewer respiratory symptoms even when age, sex, cigarette smoking, and amount of cannabis used were taken into account. Age, sex, cigarettes, and amount ofcannabis also had significant effects. The number of cigarettes smoked and amount of cannabis used interacted to create worse respiratory problems. A significant interaction revealed that the impact of a vaporizerwas larger as the amount of cannabis used increased. These data suggest that the safety of cannabis can increase with the use of a vaporizer. Regular users of joints, blunts, pipes, and water pipes might decrease respiratory symptoms by switching to a vaporizer.

Clin Pharmacol Ther. 2007 Nov;82(5):572-8.
Vaporization as a smokeless cannabis delivery system: a pilot study.
Abrams DI, Vizoso HP, Shade SB, Jay C, Kelly ME, Benowitz NL.
Abstract
Although cannabis may have potential therapeutic value, inhalation of a combustion product is an undesirable delivery system. The aim of the study was to investigate vaporization using the Volcano((R)) device as an alternative means of delivery of inhaled Cannabis sativa. Eighteen healthy inpatient subjects enrolled to compare the delivery of cannabinoids by vaporization to marijuana smoked in a standard cigarette. One strength (1.7, 3.4, or 6.8% tetrahydrocannabinol (THC)) and delivery system was randomly assigned for each of the 6 study days. Plasma concentrations of Delta-9-THC, expired carbon monoxide (CO), physiologic and neuropsychologic effects were the main outcome measures. Peak plasma concentrations and 6-h area under the plasma concentration-time curve of THC were similar. CO levels were reduced with vaporization. No adverse events occurred. Vaporization of cannabis is a safe and effective mode of delivery of THC. Further trials of clinical effectiveness of cannabis could utilize vaporization as a smokeless delivery system.