For starters I think this is an awesome business model. I have been using a virtual assistant out of the Philippines through UpWork for the last 9 months. My first suggestion would be to try it for yourself.. Get a feel for the business model and maybe see what can be tweaked or modified to provide a better experience for the customer. After doing that THEN you should consider a good marketing strategy before entering. I looked at 12 different services for virtual assistants before I settled on UpWork, so whatever you build is going to need some good exposure and SEO!