So, after going through all 300 vehicles and making this call, what will the final feedStatusData.ToVersion get me?
The the result at index 299 would be the toVersion of the status data record for last result in the feedStatusData.Data or version 0 if there is no data found.
Is it tied to the vehicle in some way?
It is the last record token for status data that was returned using the search provided. The token is not tied to the vehicle, the status data is. There is no information about the search you used encoded in the version.
If I make a GetFeed StatusData call later using that version number but don't pass device search, will I get what I think I should get: all records for all devices posted since the last device-specific feed I had requested?
If you don't pass in a device search, you will get back all records (for any device) that have been added (or possibly changed for calculated data - ex exception events) since the version you supply.