Because whatever you want to label Vietnam, in terms of business what matters is that it's fairly deregulated and open for international companies, and has been

Because whatever you want to label Vietnam, in terms of business what matters is that it's fairly deregulated and open for international companies, and has been for decades, offers significant tax breaks for hitech manufacturing, and has a cheap workforce and an established supply chain. India or Thailand might well also be competitive, but it's not a given they're any more attractive. If you mean to suggest there'd be the same risks as China, consider that Vietnam has consistently kept a certain distance to China, and have shared concerns with the US over Chinese expansionism. They're not particularly ideologically close, and politically there are few potential situations where deteriorating relationships between the US and China will negatively affect US-Vietnam relations, and some where it might improve them (e.g. if the issues in the South China sea escalates).