Celebrity directs you to www.visacentral.com/celebritycruises. When you answer the questions they correctly say you need a visa for vietnam and want to have you do it and pay the visa fee plus a processing fee of $75. If you look here https://visacentral.com/additional-client-info it says that Celebrity will obtain these visas on your behalf.
Celebrity Guest Info.
[CLIENT_NAME] is pleased to offer its customers the expertise and experience of VisaCentral. Please see below for important information that may relate to your tour.
Visas on arrival: For U.S Citizens, it is not necessary to complete visa applications through VisaCentral for the following destinations as the cruise line will be obtaining these visas on your behalf: Kenya*, Indonesia, UAE, Bahrain, Egypt, Vietnam, Cambodia, Myanmar, Mozambique, Jordan, Oman, Papua New Guinea. If a fee is required to obtain these visas, it will be charged to your onboard account. * Please note, if you are doing a pre-cruise tour in Kenya, a visa is required. This visa can be obtained in advance through VisaCentral or upon arrival at the airport. If you hold a foreign passport, you must confirm with the cruise line if it will be able to secure visas for you onboard the ship for your nationality.