When doing streaming there is limited pre-load. The video is getting loaded in the player buffer. I think that the pre-loading that you are talking about is related to progressive download, which is not a feature supported by Wowza.
In order to avoid the pre-loading (buffering) behavior you are experiencing, you need to make sure your content bitrate is one supported by the client internet availability.
As a remark, you will need to have your VOD assets already encoded into different versions of the same file, each version of the file being encoded using different bitrate values. Also, you need to make sure that all different versions of the same video clip are properly key-frame aligned in order to have a proper adaptive bitrate streaming implementation.