"However, can VanGoethem face justice in U.S. for impaired driving causing manslaughter?"
No, he should be sent back to Romania.
On the basis of the information we've been given, we don't know if the marine was driving drunk, only that he had been drinking alcohol. Do we know the law in Romania? By what standard would the marine be measured? What are the rules of evidence there? Is the accused allowed an attorney? If found guilty, what is the penalty? Would you agree to have a serving member of our armed forces tried and sentenced according to sha'ria law, for instance?
IF drunk driving was the cause of this unfortunate death, then the marine should be held accountable in the United States and punished to the full extent of the law, if guilty. To do otherwise is to set a very dangerous precedent.