I don't really like Pass
throughs because they are just encourage people to test the safety overcharge circuit by definition - considering they are usually plugged in all they time. To make matters worse, people often leave them plugged in overnight, unattended which is a big no no. That said the KGO VV may serve your purposes as it is a decent passthrough PV. It uses a USB to mini USB cord to charge (which can easily be plugged into most modern vehicles, or a cigarette lighter to USB adapter) that it comes with (no extra purchases required), and it is also has a battery which will allow you to use it without the plug. You can charge a KGO VV and use it at the same time. I don't really think you'll ever use the 6v maximum that it has to offer, but for the price in a passthrough you're not likely to beat its functionality. I know Hoosier sells a lot of them ($25 + S&H).
If you just want a regular old small PV then an eGo Mini, or the various keychain models are as small as it gets. If you need the VV functionality, I personally think that a Twist or a Spinner might bet a better choice over the KGO VV if passthrough isn't truly what you are looking for but all eGos do tend to have durability issues (especially when dropped) so there's that to consider as well.