There is no doubt that there are many chemicals present in the brain during trauma that aren't there or are there in lesser quantities when the person is not under stress. One traumatic event can physically alter the amygdala which controls our emotional responses. It, of course, acts in concert with other areas of the brain as well.
If a so called 'vaccine' were or could be developed to prevent the pre-disposition of combat PTSD, in my opinion it would only make it more difficult for veterans with combat PTSD to get the treatment that they need. Both the military and VA's would use it as an excuse to refuse claims for combat PTSD based on the fact that people were 'vaccinated' against it.
There are those, no doubt, that have a pre-disposition for mental conditions of all kinds including PTSD and combat PTSD. I don't think that you can give anyone a drug and send them to war with the hope that after experiencing what you do in combat that they wouldn't be changed forever. I'm a combat vet and I can tell you that you can not look upon the face of war and not be changed by it.
I agree with Anthony in that it's not called Big Pharma for nothing.