If you're just looking to apply the class when the visitor group is being impersonated in edit/preview mode then you could do something like the following as the ID of the impersonated visitor group is passed in the querystring:

var impersonatedVisitorGroupId = Request.QueryString["visitorgroupsByID"];
Guid visitorGroupGuid;
var className = string.Empty;
if (!string.IsNullOrEmpty(impersonatedVisitorGroupId) && Guid.TryParse(impersonatedVisitorGroupId,out visitorGroupGuid))
{
    var visitorGroupRepo = ServiceLocator.Current.GetInstance<IVisitorGroupRepository>();
    var vgName = visitorGroupRepo.Load(visitorGroupGuid).Name;
    className = "vg-" + Regex.Replace(vgName, "[^a-zA-Z0-9-_]", "-").ToLower();
}

The code above gets the visitor group ID from the query string, gets the name of the group from the IVisitorGroupRepository and converts it into a class name which you could then pass back into your viewmodel to render into the class attribute of the body tag on the page.

@Paul - thanks for the reply

I like the approach but am confused about where that code would go.
Are you implying I should write an 'edit mode' controller that can evalutate this logic?

Initially I thought I could do this in my CookieCriteria class inside of the IsMatch method but the CookieCriteria class does not get called when I toggle through the eye toggle

I'm assuming you're looking to add the class to the body tag in the layout file used by your pages. I'm also assuming you're using a view model to pass non-page-specific data to your view. If both of those are true then you could add the className as a property to the view model and that logic could go into the same place as the logic to populate your view model. In the case of an alloy site that would be the PageViewContextFactory. If it's just relevant to one page type then it could go into the controller for that page type.