You will certainly incur overages using the VZ Navigator with a 25MB data plan. I would imagine that 25MB might be just enough to navigate and download maps for one trip. Map images are data intensive, if you plan to use it, I would switch to the unlimited data plan.
The VZ Navigator uses MB usage when you key in a address and it is being searched. Once turn by turn directions are downloaded no other MB will be used. Average MB used is 1 MB per 1-3 searches depending on search.