Do Stroyboards, hire a concept artist, do previs… and anything else that you can do to get your vision across as early and clearly as possible. VFX people do not "wing it"... They are careful planners and strategists... They will cling to any tangible piece of direction you can give them. The more information you put out the better the VFX team can disseminate information to the rest of the crew.
Tom Turnbull