I'm no expert, but I have owned a few genuine Vista DVDs so far. For craps and giggles I've used Beyond Compare to compare at least two different images that I personally made of different Vista discs. I know I compared a Vista Business disc that Microsoft was giving away at trade shows and an OEM Vista Home Premium disc. I'm also pretty sure I've compared my retail Vista Home Premium disc. They were all IDENTICAL with the exception of around ~10 (or less) blank characters (ascii code 255 I think) at the very end of at least one of the discs.
- To put this a different way: The 64 bit image I have is 3,796,500,524 bytes, when I've compared disc images approximately the first 3,796,500,514 bytes were identical between all discs. The tiny portion that wasn't identical was only a few blank characters at the very end of the file.
[edit]Sorry, I forgot one pretty important detail: the discs only have the 32 bit versions OR the 64 bit versions. To be prepared for all possible installs you'll need two discs, a 32 bit and a 64 bit.[/edit]

Why I mention this is to let you know that if you can get ahold of a real Vista disc you should be all set, no matter if the key you have is for an OEM version, retail version, Home Premium, or Ultimate.

I just remembered an issue I had once while activating Vista on an Acer laptop that came with Vista Home Premium with service pack 1... None of my discs have service pack 1. IIRC the key on the bottom of the laptop would allow me to install Vista Home Premium OEM but when I got finished it wouldn't activate for some odd reason. In fact, I think I now remember having an issue where I couldn't get it to activate over the phone either. Then I realized that it may be the lack of having SP1 installed that was holding me up. After installing SP1 it activated over the internet without any more issues. Perhaps you're having a similar problem? If the code is for SP1 and you only have the... uh... "SP0" disc you may be in business. If the code is for SP0 and you only have an SP1 disc you might have an issue, but this is a scenario I haven't yet tried.

I'm pretty sure I called Microsoft tech support (not just the phone activation people) when I was having this problem. I even told them I was using a burned DVD (I quickly blurted out "as it says I'm allowed to do in the EULA"). They didn't seem to have a problem with it. As I eluded to before, every time I've called the people on the other end have been pleasant. Unfortunately this particular call didn't give me the answer I needed (I had to figure that out on my own) but they were nice and were trying to be helpful.

I'm not certain, but I would think there would always be a phone activation option (what if you didn't have internet access?). If all else fails disconnect from the internet (turn off the Wi-Fi if you can) and try activating again. Windows should give you the phone option when it figures out that there isn't an internet connection available.

I've done the phone activation many times. The Microsoft voice recognition system is actually very good. Usually after you say the activation code you'll be connected with an operator who will ask a single question which is something like "onto how many computers is this copy of Windows installed?". You just tell them how many (it should be just one of course) and they'll give you a new code to type into the comp. It almost always takes between six and seven minutes.