Cheaper price (1 month single entry: 6 USD, 1 month multiple entries: 8 USD, 3 month single entry: 15 USD, 3 month multiple entries: 20 USD) is for non-rush visa tourist request and applies to passport from the following countries:
Albania, Andorra, Anguilla, Antarctica, Antigua and Barbuda, Argentina, Armenia, Aruba, Australia, Austria, Azerbaijan, Bahamas, Barbados, Belarus, Belgium, Belize, Bermuda, Bhutan, Bolivia, Bosnia and Herzegovina, Brazil, British National Overseas, British National Overseas - BNO, Brunei Darussalam, Bulgaria, Cambodia, Canada, Cayman Islands, Chile, China Mainland, Christmas Island, Cocos (Keeling) Islands, Colombia, Cook Islands, Costa Rica, Croatia, Cuba, Cyprus, Czech Republic, Denmark, Dominica, Dominican Republic, East Timor, Ecuador, El Salvador, Estonia, Falkland Islands (Malvinas), Faroe Islands, Fiji, Finland, France, French Guiana, French Polynesia, French Southern Territories, Georgia, Germany, Gibraltar, Greece, Greenland, Grenada, Guadeloupe, Guam, Guatemala, Guernsey, Guyana, Haiti, Heard Island and Mcdonald Islands, Honduras, Hong Kong (China), Hungary, Iceland, India, Indonesia, Ireland, Isle of Man, Israel, Italy, Jamaica, Japan, Jersey, Kazakhstan, Kiribati, Kosovo, Kyrgyzstan, Lao, Latvia, Liechtenstein, Lithuania, Luxembourg, Macao (China), Macedonia, Madagascar, Malaysia, Maldives, Malta, Marshall Islands, Martinique, Mauritania, Mauritius, Mayotte, Mexico, Micronesia, Moldova, Monaco, Mongolia, Montenegro, Montserrat, Myanmar, Nauru, Nepal, Netherlands, Netherlands Antilles, New Caledonia, New Zealand, Nicaragua, Niue, Norfolk Island, North Korea, Northern Mariana Islands, Norway, Palau, Panama, Paraguay, Peru, Philippines, Pitcairn Islands, Poland, Portugal, Puerto Rico, Reunion, Romania, Russia, Saint Barthelemy, Saint Helena, Saint Kitts and Nevis, Saint Lucia, Saint Martin, Saint Pierre and Miquelon, Saint Vincent and the Grenadines, Salomon Islands, San Marino, São Tomé and Príncipe, Serbia, Seychelles, Singapore, Sint Maarten, Slovakia, Slovenia, South Africa, South Georgia and the South Sandwich Islands, South Korea, Spain, Suriname, Svalbard and Jan Mayen, Swaziland, Sweden, Switzerland, Taiwan (China), Tajikistan, Thailand, Tokelau, Tonga, Trinidad and Tobago, Turkmenistan, Turks and Caicos Islands, Tuvalu, Ukraine, United Kingdom, Uruguay, Uzbekistan, Vanuatu, Vatican, Venezuela, Virgin Islands, British, Virgin Islands, U.S, Wallis and Futuna, Western Sahara, Western Samoa

The citizens of following countries are eligible for E-VISA or Vietnam Visa On Arrival (1-month tourist visa) but the price is higher (NO paper nor document required):
Algeria, Angola, Bahrain, Benin, Botswana, Bouvet Island, Burkina Faso, Burundi, Cape Verde, Central African Republic, Chad, Comoros, Djibouti, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Guinea, Guinea Bissau, Iraq, Ivory Coast, Jordan, Kenya, Kuwait, Lebanon, Lesotho, Malawi, Morocco, Namibia, Niger, Oman, Palestine, Papua New Guinea, Qatar, Rwanda, Saudi Arabia, Senegal, Somalia, South Sudan, Sri Lanka, Sudan, Tanzania, Togo, Tunisia, Uganda, United Arab Emirates, United States, Yemen, Zambia, Zimbabwe

The citizens of following countries are eligible for E-VISA or Vietnam Visa On Arrival but the price is higher with document required (Hotel Booking, Return Air-ticket & Passport scan). Visa will be issued for the period of staying as in tickets and hotel booking:
Afghanistan, Bangladesh, Cameroon, Congo Democratic Republic, Congo Republic, Ghana, Iran, Liberia, Mali, Mozambique, Pakistan, Sierra Leone, Turkey

Please note that the following countries are not eligible for E-VISA or Vietnam Visa On Arrival, they will need to contact the embassy or consulate of Vietnam to apply for visa prior their trip to Vietnam:
Libyan, Nigeria, Syria