Vietnam service can butterfly everything. He won't be a teetotaler probably and he could end up being like his brother, who died young.
But assuming Trump goes to 'Nam and serves in an okay matter, I agree with the assessment he would build himself to be a war hero and such.
I presume he would start his political ambitions sooner, which ironically would likely result in failure, and having failed at politics, he'll focus on real-estate which made him and his father so much money.
Trump is Prez because of him being in the right time, right place. Nam changes that.