Sure, It's great that you are planning to explore Vietnam. Here are some tips for your travel:

Best Time to Visit:
Hanoi: The best time to visit is from October to December when the weather is pleasantly cool.
Hoi An: The best time is from February to July, avoiding heavy rainfall and hot summer.
Nha Trang: The city is best visited from January to August.
Ho Chi Minh City: The dry season from December to April is the best time.

Route and Accessibility:
You can start from Hanoi, take a flight or train to Hoi An. From Hoi An, travel to Nha Trang by bus or train then finally head towards Ho Chi Minh City. This route will ensure a convenient journey.

Altitude and Acclimatization:
These cities are at sea level, so you don't need to worry about altitude sickness.

Accommodation and Booking:
All these cities have a wide range of accommodations catering to different budgets. It's preferable to book in advance to secure your spot, especially during peak tourist season.

Clothing and Essentials:
Pack lightweight, comfortable clothing as Vietnam generally has a tropical climate. Don't forget to pack swimwear for Nha Trang's beaches and sun protection. Always carry a reusable water bottle and a good map.

Health and Safety:
Be aware of traffic rules, particularly in hectic Ho Chi Minh City. Check your vaccinations before travel. Eat and drink from reliable sources to avoid foodborne diseases.

Local Etiquette and Respect:
Vietnamese people appreciate modest dressing, respect for their customs and traditions. Remember to remove your shoes before entering temples or someone's home.