A couple things to keep in mind when merging Visio drawings:
1: Visio performance drops with the number of shapes in a drawing
2: What looks like a single shape can actually be dozens or even hundreds of shapes and grouped shapes.
3: Best practices are to keep the number of shapes and sub-shapes in a visio drawing to as few as possible.
The best way to merge a Visio drawing with D-Tools is to follow the instructions in this post
http://www.d-tools.us/3563-post11.html and save as a WMF or DWG before bringing in.