The wedding will be held at Amiana Resort, which is about a 20 minute drive north of downtown Nha Trang. Some guests are opting to stay there for the wedding weekend, and to stay at a hotel downtown for the other days of their trip. There are lots of really good hotel and Airbnb options around Nha Trang.
As soon as possible! Flights across the pond tend to get more pricey as the date gets closer, so try to secure those tickets soon. Your best bet is likely to book a flight from the US to Saigon (aka Ho Chi Minh City), then separately book a flight or train from Saigon to Nha Trang. Feel free to reach out if you need help!
US Citizens are required to get a visa to travel to Vietnam. Make sure you print it out once you get the eVisa! You should do this ahead of time on the website: https://evisa.xuatnhapcanh.gov.vn/en_US/web/guest/khai-thi-thuc-dien-tu/cap-thi-thuc-dien-tu Also, make sure you will have at least 6 months before your passport expires at the time of travel. More info: https://travel.state.gov/content/travel/en/international-travel/International-Travel-Country-Information-Pages/Vietnam.html