Visa advice from Vietnamese or UK Expats

Hi.

I am looking to get some advice from Vietnamese who have visited the UK, or UK expats who have been through the process of helping Vietnamese get a UK visit visa.

I was looking for advice on trying to get my friend a visit visa for the UK and was wondering if anyone has experience or advice on this subject please?  We are trying to get a 2 week holiday visa arranged for the UK for this summer.

Here is some info about my friend  -
•    She has a passport for about 18 months, but has never had any foreign travel.
•    She has strong links to Viet Nam which should help prove she is not a flight risk.
•    All her family live in VN and has no other friends or family in the UK
•    She has a job with foreign venture capital company, which she will return to.
•    She has just completed a University course, and currently attending other college course for advanced Japanese, which she will return to.
•    My friend and I have known each other for over 3 years and I have been to Viet Nam 8 times.

Some questions I was looking for advice on please –

•    Do we stand a better chance going through an agency to get the Visa? (or are they a rip off?, I hear using an agency is like giving someone your watch and paying  them to tell you the time.)
•    If an agency is the best option, does anyone know of a good agent, and how much they will cost?
•    Does she need to provide enough evidence that she will return to VN after her visit to the UK? Such as details of her employment , cash in bank account etc?
•    If cash in the bank is required, then how much is needed?
•    Is there a greater chance of being given a Visa if she has travelled to other countries previously?
•    If previous foreign travel is required, will a short trip to Singapore and Thailand be enough?
•    As my friend will be visiting me in the UK, do I need to provide my details to help support the application? Evidence of our relationship etc
•    Do I need to write a letter to sponsor her? Provide details that I can financially support her while she is on holiday in the UK? (none of this is a problem as I am in full time employment in a senior position and have significant funds in my bank account)
Any other advice would be appreciated.

Thanks

FC