U.K. visa application for my Vietnamese fiancé

For the UK, sorry, you will not find find.
That is outsourced to an indian company. You friend will have to manage that alone
https://www.vfsglobal.co.uk/sa/en

I've sponsored and helped many Vietnamese nationals get UK visas.

Using an agent seems to be the default Vietnamese response to doing anything. There's a reason why, agents facilitate bribes to get things done when dealing with their own Government. Recently discovered this when my GF was trying to secure a laisse passe for a trip to China. The young lady at the counter said she would never ever...ever get one. My GF replied that the agent sitting across the road could get her one same day but it would cost her more than applying herself to which she was told to bring that person to talk to her. So sure enough agent goes in and waits specifically for  her and walks out with her laisse passé. She must have been early 20's...so young, so corrupted and so evil.

I digress. Not implying your using an agent for these reasons... as they can't really help you in that way with  UK immigration. just making a point that it's always a Vietnamese person first response to use an agent to do everything.

I'd say, do it yourself. Include a covering letter addressing their concerns about your previous application. Show sincereity and you shouldn't have a problem. Drop me a message if you want specific advise on what points they failed you on.

I've done  7 applications, 4 different people, 4 successes, 3 rejections. I would never  use an agent for something you can do yourself for free and do a better job.