Scott, I think you probably have the chest-spasm traumas in the right order, at least on average: cough, sneeze, vomit. Most of us suffer through intentional and accidental/spontaneous coughs in the immediate post-op period, and try like mad to avoid sneezing, which is generally even more explosive and potentially painful.
In my 1+ years on this forum, I've only heard of TWO patients who actually broke the stainless-steel wires that most of us get to hold our sternum halves together, and they BOTH did it from vomiting -- at least one of them after drinking too much. Sneezing is nasty, but I haven't read about anybody breaking anything from it.
There are various drugs that can help avoid each of those three kinds of spasms, and I suspect that even we relatively drug-shy folks should consider reaching for them to avoid too much stress to those wires, and our early sternum healing. (Me, I developed a nasty cough post-op, and ordinary cough suppressants wouldn't suppress it, so I got a prescription for a codeine cough syrup that got rid of it properly. My first sneeze surprised me somewhere around 5 weeks post-op [my "Norm's story. . ." blog thread remembers the timing better than I do!], and by then it wasn't too traumatic.)