Most GPRS-enabled vehicle trackers can be configured to send the position data periodically to a remove server. The protocol and the details of the connection will vary a lot between different manufacturers. However in most cases, you should be able to configure the tracking device with the IP address and TCP port of your server, and then you should set up an application at that endpoint that listens for TCP connections from the tracking devices. This back-end application can then push the data it receives into a database, from where your ASP.NET web application would be able to retrieve the data.
The first thing you should check is the documentation of the tracking device, if this was already chosen by your client. The serious brands will even provide working examples and demonstrations.
When you solve that part, and you start getting the data in the database, it will be a matter of serving the information from the database through your ASP.NET web application. To have the vehicles moving in realtime, you should probably look into AJAX or long-polling.
As for Google Maps, note that the you cannot use the free version for vehicle tracking. Article 10.9.C of Google Maps API Terms and Conditions explicitly prohibits using the standard (free) Google Maps API for non-public fleet management and asset tracking. You would need the Google Maps API Premier to legally use Google Maps for these kinds of applications.
As an additional side-note, according to one unofficial source (dated April 2008), the Premier API for vehicle tracking would cost USD 10,000 per year, which entitles you to track 100 vehicles. If you exceed the 100 vehicles, you would need to add USD 24 per additional vehicle per year.