We try to keep the Payment plugin simple to use by avoiding such fine-grained configuration of Payment Plans for individual applications.
For a bit more complex scenareo such as yours where you have users with different subscriptions in an application that you need to charge differently for, we can provide another solution for you.
There is a plugin named UserSync (the plugin is currently in beta stage), see http://gpsgate.com/blogComment/usersync_plugin
UserSync allows you to automatically copy users from a particular tag in a source application to another application.
To solve your particular scenareo you would create one Tag per user subscription type in your original VehicleTracker application and set up a UserSync rule that will copy users of a single tag into a separate "payment application". You can then activate different Payment Plans that you have created with different Product Type and unit price for those different "payment applications".
That is, you would not activate a Payment Plan for your original application. Instead you would activate different Payment Plans for the different "payment applications" updated by UserSync based on the subscription tags in the original application. Note that for the UserSync rules you would have to check the checkbox "Synchronize when a user is deleted" so that you do not keep removed users in the "payment applications".
One drawback with this approach is that you will not be able to receive login overdue notification messages when there are overdue payments in a "payment application" when logging in to the original VehicleTracker application. However it might not even be the case that you need such login overdue notifications. Email notifications can still be used.