I've read in the magazines that Microsoft is going to stop updates for Vista, will this leave my computer open to attack, I use AVG free.

Yes they will End of Life it at some point in the future.
This is perfectly normal and recently happened to XP.
It's all part of the upgrade path to push you into getting a more reliable machine and OS.

So does that mean I need to buy another computer, or I could be hacked.

You could be hacked whatever you do anyways..

No you do not need to buy another computer. But I would recommend upgrading your PC to Windows 7. A Vista machine should run Win 7 without a problem. As long as your Anti-Virus program is up to date, you have reasonable protection, but no protection is foolproof.

As Ben noted, all versions of Windows eventually become "sunsetted". This means that Microsoft will no longer work on updates to the OS. Vista Support will continue to April 2017, so there is no rush for you.

But Win 7 is a better, more stable version of Windows.

As noted, There are many ways to "hack" a computer. An anti-virus is only one protection. You need a firewall and anit-malware protections as well. What exactly do you fear happening?

I use the free AVG am I protected, or do I have to have one of those paid versions?

I use the free AVG am I protected, or do I have to have one of those paid versions?

I am not a big fan of free anti-virus tools. New viruses come out daily. To be protected, the anti-virus company needs to constantly research, catch, test and release new signature files regularly. This takes money So I don't see how the free ones can do it.

Free AVG does offer some protection as long as they keep updating it and you keep yours updated. It's certainly better than nothing. However, some security holes cannot be plugged by antivirus software. They need to be fixed by an update to Windows. If this type of security hole in Vista is discovered, you will be vulnerable to hacking.