Hold the Windows key and press P (for "Project"). That will bring up a menu that will allow you to choose your display setup. If you keep holding the WinKey down, you can use repeated presses of the "P" key to cycle through options.
Also, I would check the documentation for your adapter. If you're going to composite video, that's limited to 480i, so the adapter might require you to lower the resolution being sent from your VGA output in order for the adapter to work. If you need to do that, it would be under Control Panel > Display. There's probably a faster way you can get there by right-clicking your background image, but I honestly can't remember whether Vista's right-click menu there works more like XP or Windows 7, hence recommending the Control Panel route.
WinKey+P isn't a model-specific combination; it's a Windows combination, but maybe that wasn't introduced until Windows 7. In terms of your system, look on the F keys for an icon of a display, and then try holding the Fn key and pressing that key. Fyi, it's always helpful to name the specific model of your system. "Vostro" is an entire model line that has had several models over several years (like Inspiron, Latitude, Precision, etc.), so you'll generally get more pertinent advice if you specify the exact model.
If you can't find that key, you should be able to go to Control Panel > Display and see a second display listed there if you have the adapter connected to the PC and the TV. If so, you should be able to select that and then choose to extend or mirror your desktop to that display, or display exclusively on the external display (which will turn off the built-in panel).