Surely! It seems like you are preparing for an exciting journey across Southeast Asia. Here are some detailed tips for visiting Vientiane, Phnom Penh, Ho Chi Minh City:

Best Time to Visit:
The optimal time to visit these cities is during the dry season:
November to March. During these months, the weather is relatively cool and comfortable, which makes sightseeing and outdoor activities pleasant.

Route and Accessibility:
You can take a direct flight to Vientiane from most major cities. From Vientiane, you can either travel by bus or fly to Phnom Penh. Once you're done with exploring Phnom Penh, there are numerous direct flights you can take to Ho Chi Minh City.

Altitude and Acclimatization:
All these cities are situated near sea level, so altitude sickness shouldn't be an issue. However, keep yourself hydrated to beat the heat.

Accommodation and Booking:
All three cities have a range of accommodation options suitable for different budget types. It's highly advisable to book your accommodation and transport in advance to avoid any last-minute hassles, especially during the tourist season.

Clothing and Essentials:
Due to the warm climate, light and breathable clothing is recommended. Carry sunscreen, mosquito repellent, and a hat for extra protection. Comfortable shoes are important due to the amount of walking you'll likely be doing.

Health and Safety:
Stick to bottled water to avoid any health issues. Ensure that your routine vaccinations are up to date and consider getting those recommended for travel in Southeast Asia. Street food is a highlight in this region, but choose wisely to avoid an upset stomach.

Local Etiquette and Respect:
In all these cities, always remember to respect local customs and traditions. Dress modestly when visiting religious places, and always ask for permission before clicking pictures of locals. In Vietnam and Laos, a soft handshake is the usual greeting, while in Cambodia, locals greet by placing their hands together in a prayer-like gesture.

These tips should help ensure a seamless travel experience. Enjoy your trip!