Can anyone who used this agency (http://www.bestpricevn.com) share feedback re. their experience? Thanks.

Never hear of them. Where id you get the recommendation?

A couple of local VN agencies that are often recommended by Fodorites: Tonkin, Vietnam Stay

I used them to book a river cruise on the Mekong from Chiang Kong in Thailand to Luang Prabang in Laos on Luang Say Cruises, their price was less than the boat operator.

Everything was fine, all booked online and and easy to pay. Email contact was good.

They did ask for a copy of my Vietnam visa - we weren't going to Vietnam, so perhaps lacking attention to detail...

Thanks. They were suggested by an acquaintance of mine but wanted to get more info from those who used them.

Try Indochina adventure in Hanoi.
Its owned by a Scottish guy.
They are professional, honest, don't try to sell you more expensive options if you are clear about what you want. their prices are very fair.
We got a bad deal from another travel agent(don't remember his name - across the street from Indochina, American guy married to a local woman). Indochina fixed the bad deal and made the remainder of our trip amazing.