Yes, a valid visa for each nation is one of the basic admission criteria. 

Cambodia Visa 

Travelers have to get a Visa on Arrival (VOA):

• Available at: Major international airports (Phnom Penh, Siem Reap, Sihanoukville) and certain land borders
• Validity: 30 days (Tourist Visa - T class)
• Requirements: Valid passport (at least 6 months validity), 01 passport-sized photo, Visa fee (around US$ 30 - $35), and completed visa application form (available at the airport/land border).

Vietnam Visa

You should apply e-Visa in advance at: https://evisa.gov.vn/

Or else, you can contact BestPrice Travel to apply e-Visa (US$ 40/person) with simple procedures and time-saving.

Cruises from Cambodia to Vietnam usually last between 4 and 9 days, depending on your chosen route. The start/end points are often Phnom Penh/ Siem Reap and Ho Chi Minh City/ Can Tho.

• Shorter trips (4 - 6 days) focus on Phnom Penh and the Mekong Delta.
• Longer journeys (7 - 9 days) add explorations around Siem Reap with cultural stops.

A Cambodia to Vietnam cruise typically costs from US$ 150 to $325/person per day. The price varies based on the length of the voyage and the level of service provided. 

The following is the average price for various itineraries (excluding transfer fees): 

• 4 - 6 days: From US$ 591 - $1,812/person.
• 7 - 9 days: From US$ 1,514 - $2,604/person.

Here are the top Cambodia-Vietnam cruises, with the best price and travelers' reviews (the price does not include transfer service): 

• Victoria Mekong Cruise 5 days: Phnom Penh - Saigon (from US$ 1,092/pax) 
• Mekong Princess Cruise 6 days: Phnom Penh - Saigon (from US$ 1,812/pax)
• Mekong Navigator Cruise 8 days: Siem Reap - Saigon (from US$ 2,402/pax)
• Heritage Line Jayavarman Cruise 8 days: Siem Reap - Saigon (from US$ 2,403/pax)
• Heritage Line Jahan Cruise 8 days: Siem Reap - Saigon (from US$ 2,604/pax)

Here are some of the top things to do after you visit Vietnam (for 2 - 3 day stays).

Disembark in Saigon:

• Join a city tour: War Remnants Museum, Notre-Dame Cathedral, and Central Post Office.
• Shopping in Ben Thanh Market – local food and souvenirs.
• Explore Cu Chi Tunnels.
• Nguyen Hue Walking Street – lively at night.

Disembark in Can Tho:

• Cai Rang Floating Market – best early morning (start ~5–6 AM).
• Boat tour of small canals & fruit gardens.
• Ninh Kieu Wharf – riverside dining and walking.
• Ong Pagoda – a small but vibrant Chinese temple.