Is it possible for someone with vulnerable (soft) plaques to have a negative treadmill stress test, but still be in danger?

Yes: It's entirely possible - this is, in fact,the explanation for how someone can have a normal stress test and then have a heart attack hours, days, or weeks later: presumably there is no serious blockage at the moment of the stress test, but they rupture a plaque and occlude the vessel at a later time. This is a sad reality. We have no "crystal balls" to predict the future.