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As far as I can tell, the Vietnamese government does not publish any list (at least not in any language I understand) of authorised travel agents to issue a pre-approval VoA letter. I can vouch from personal experience that this unusal route of obtaining a letter from a travel agent is legitimate, however there is no way of you knowing if you have received a valid letter until you arrive at the border.
If you wish to travel with more certainty of entering Vietnam, you have two options:
Get an e-visa from https://evisa.xuatnhapcanh.gov.vn/. This is an actual Vietnamese government site, that I found via the British FCO linking to it. Note that there are many commerical travel agent sites masquerading as offical government sites.
Apply for a traditional paper visa from your nearest Vietnamese Embassy. Note that this has the additional advantage that it can be used at any entry point, not just certain airports, and may have greater flexibility. Again, the Vietnamese government does not maintain a readily accessible list of the websites for these. I suggest you find their contact details via the local government where you are. To the best of my knowledge, the Vietnam does not care about citizenship in relation to which embassy you apply to.