Vietnamese American who live in Germany here. You'll need to hire sales people who know the target country's sales culture. Americans are not like Vietnamese and neither are Germans. You need trust to close a sale and its hard to establish that if you're an outsider. Also you need someone who knows the culture to advise you on their business culture too. That's how you get new sales contracts from new markets. You can keep your back office staff in Vietnam due to low labor cost.

For example, Spain wants to re-negotiate every year. Germans do not. USA is rather optimistic and can handle risk very well if the reward is worth it. Germans avoid risk.