Upvote:4
I have not used this program myself, however as long as you meet the conditions (and there are more than just what you've stated) then you should not expect to have any problems.
Many airline check-in staff will be aware of the program, however even if they are not it doesn't matter. Airlines verify entry requirements for a country based on systems such as "Timatic" which provide details of what a passenger requires to enter a country, based on their nationality. The Timatic entry for a Vietnamese citizen entering Ireland from Timatic is included (in part) below. The airline staff will consult this output, and if you meet the requirements listed you will be allowed travel - regardless of whether they are specifically aware of the visa waiver program.
Relevant details from Timatic are :
Visa Exemptions:
Nationals of Viet Nam with a short stay visa issued by the United Kingdom if they have first entered the United Kingdom and been granted a stay of 180 days in the United Kingdom. They are visa exempt for a maximum stay of 90 days in Ireland (Rep.) or until the end of the period of stay granted in the United Kingdom, whichever is shorter. Information: Passengers arriving in the United Kingdom, including Northern Ireland, and traveling to Ireland (Rep.), the visa waiver programme will apply and only a visa issued by the United Kingdom will be required. The visit to Ireland (Rep.) must be within the stay granted within the United Kingdom. Passengers are permitted to travel to a third country before traveling to Ireland (Rep.) if the visa issued by United Kingdom is still valid. Furthermore, re-entry into Ireland (Rep.) from a third country is permitted if the visa issued by the United Kingdom is still valid and period of the stay granted in the United Kingdom is still valid. Passengers with a long term visa must visit Ireland (Rep.) within a period of the current stay granted in the United Kingdom.
Passengers arriving first in Ireland (Rep.) and traveling to the United Kingdom, including Northern Ireland, and then returning to Ireland (Rep.) will require separate visas issued by Ireland (Rep.) and the United Kingdom. However the visa issued by the United Kingdom will be accepted under the visa waiver programme for the return journey to Ireland (Rep.).
Information: Not applicable to short stay visas issued for the purpose of transit, marriage or to enter into a civil partnership.