There is no readily available Visio stencil that can be used for a RSA enVision appliance, on the RSA website.

The RSA enVision Hardware Setup and Maintenance Guides, available on the RSA SCOL website does have pictures of the appliance hardware.  Can use this image in the Visio diagram.

As the RSA enVision Hardware is in fact a rebadged Dell PowerEdge 2950 or Dell Poweredge R710, you can use the Dell Visio stencil for this sort of hardware.
You can download the Dell Visio stencil for this sort of hardware, from various websites on the Internet.

From the Visio Caf? website, see http://www.visiocafe.com/dell.htm

From the Dell website, see http://www.dell.com/content/topics/topic.aspx/global/products/pvaul/topics/en/visio?c=us&l=en

Download the Dell PowerEdge Rack Server Stencils.

 Note with the enVision appliance an additional 2-port network card is added, so it will not appear in the rear view of the default Dell Visio stencil.

Also the RSA supplied faceplate with RSA logo will not appear in the default Dell Visio stencil.