The steps are:
1. Read Visio Drawing file
2. Convert it to Scalable Vector Graphics format
3. Write new file
The problem is that old Visio Drawing file format is proprietary (and binary), so there is not much chance you will find a free library to read it (there are payed libraries). If you can get the Visio Drawing files in new (XML+ZIP) format than you can write your own library to read it:
http://msdn.microsoft.com/en-us/library/office/jj684209(v=office.15).aspx[
^].
With SVG you are in luck as it is a free format and you can easily find its definitions:
http://en.wikipedia.org/wiki/Scalable_Vector_Graphics[
^]
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