In my experience, it's all too easy when stealth-vaping to forget and blow a massive cloud now and again. That could get you in trouble and escalate the prohibition further.
I'd knuckle under with the security guards, who are only doing as they have been instructed, but take the issue to higher authority with evidence. For example:
"Current state of knowledge about chemistry of liquids and aerosols associated with electronic cigarettes indicates that there is no evidence that vaping produces inhalable exposures to contaminants of the aerosol that would warrant health concerns by the standards that are used to ensure safety of workplaces."
-- Systematic review of what the chemistry of contaminants in electronic cigarettes tells us about health risks
-- Igor Burstyn Department of Environmental and Occupational Health, School of Public
Health, Drexel University,
--
http://www.biomedcentral.com/content/pdf/1471-2458-14-18.pdf
You should be able to argue that, while you are not endangering others by vaping, the university is endangering you by forcing you to inhale second-hand smoke.
Good luck.