I would suggest that you post your requirements on freelance sites and conduct interviews to find exactly what you are looking for.
Having direct contact with your VA rather than going through an "agency" will be much easier as you can train them to do exactly what you want, rather than telling the company and hoping they explain your needs to the worker.
Spend the extra time and effort to get someone that fits your needs. There are plenty out there - interview a bunch - that's the only way you will get what you need/want
I'm going to go ahead and agree with this also.
I have hired countless freelancers for countless types of jobs. The only way to really get exactly what you want, is to do exactly what netmoney has said...interview extensively...and don't expect it to be easy. They are very good at telling you what you want to hear.
You'll need to have questions prepared that require intelligent answers from the interviewee. You don't want to ask anything that allows for a "yes or no" answer. You need to find out if they have the skills necessary for what you require and to do that effectively, you need to make sure they answer your questions with real information and not "yes man" answers.
My personal order of preference when hiring on a freelance site is:
elance.com
freelancer.com
odesk.com
Depending on the skillset required to do what you need, I'd say to ALWAYS feature the job. A featured job allows you to invite far more people than you can without featuring it. If memory serves, a featured job on freelancer allows for you to send 500 invites and a featured job on elance allows for you to invite...I can't remember...I know without featuring you're limited to sending only 30 invites. When it is featured...300 maybe? I don't remember, but it's a lot more than when it isn't featured.
Anyway, not only will you get more qualified bids by inviting that many people, but you'll also get what you're looking for at a much lower price. All the additional bids you'll receive will drive the price down. Although be prepared to have to interview 3 to 4 times as many people.