Hello! That sounds like an exciting trip across Vietnam. Here are some travel tips to help you prepare.

Best Time to Visit:
The best time to visit cities like Hanoi, Da Nang, Hoi An, and Nha Trang is during the dry season from February to April and August to October. The temperatures are more moderate and rainfall is lighter. In Hanoi, however, the winters can be quite cold.

Route and Accessibility:
Starting your journey from Hanoi is common as it hosts an international airport: Noi Bai International Airport. Local flights from Hanoi to Da Nang, Hoi An, and Nha Trang are frequent and cost-effective. You can also travel by train or bus, which offer great views of Vietnamese countryside, but it'll take a longer time.

Accommodation and Booking:
These cities offer a wide array of accommodations – from luxury resorts to budget hostels. It's advisable to book in advance, especially during peak season. Look for centrally located accommodations to easily access street markets and sightseeing locations.

Clothing and Essentials:
Pack lightweight and quick-drying clothes suitable for a tropical climate. Don't forget your swimsuit for beach locations like Nha Trang. A light rain jacket might be useful as unexpected rain showers can occur. And remember, comfortable walking shoes are a must for city exploration.

Health and Safety:
Ensure you're up-to-date with routine vaccines before your trip. Drink bottled water and drinks without ice, and eat well-cooked food. Do not forget to purchase travel insurance before your visit. While these cities are generally safe, it's always best to stay alert and watch your belongings.

Local Etiquette and Respect:
Vietnamese people value respect and courtesy. Dress modestly when visiting temples and historic sites. It's also customary to remove your shoes in people's homes and even in some traditional businesses. Bargaining is common in local markets.