It sounds like you're planning an exciting trip to Vietnam! Here are some travel tips for your journey.

Best Time to Visit:
The best time to visit Hanoi and Hoi An is from February to April and from October to December when the weather is milder. For Nha Trang and Ho Chi Minh City, the dry season from December to April is ideal.

Route and Accessibility:
Flying directly into Hanoi’s Noi Bai International Airport is the most convenient option. From Hanoi, you can take domestic flights, buses, or trains to Hoi An, Nha Trang and Ho Chi Minh City. Navigating in cities can be done via taxis, cyclos, or popular ride-hailing apps. Also, try experiencing the cities on foot for shorter distances.

Accommodation and Booking:
Accommodation options in these cities range from budget hostels to luxury resorts. Booking in advance is advisable especially during peak seasons and you can make reservations on popular websites like Booking.com, Agoda, or Airbnb.

Clothing and Essentials:
Lightweight, breathable clothes are best due to Vietnam's tropical climate. Remember to pack a good sunblock, a hat, comfortable walking shoes, and a raincoat or umbrella. A lightweight jacket for cooler evenings is also recommended.

Health and Safety:
Make sure your routine vaccinations are up to date before traveling. Street food is a major part of Vietnamese culture, but ensure it is well cooked to avoid any foodborne illnesses. Always carry bottled water, stay hydrated, and remember to apply insect repellent.

Local Etiquette and Respect:
Vietnamese people are generally polite and appreciate the same. Avoid public displays of affection, dress modestly at religious sites, and don't click photos of military installations. Bargaining is common but always do it respectfully.