I remember the main things to consider being - Type, drugs and afterbirth.
Type of birth - You say you want vaginal, but what happens if you are in labour for 24+ hours does that change things? What about if the babies or you are under stress? Basically what are your preferred alternatives if things do not go to plan. Other aspects to consider with a vaginal birth are the use of foreceps or a ventouse. Do you mind having an episitomy?
Drugs - Read up on the types of drugs available and when they can be administered at the different stages of delivery. For example, you generally can't get an epidural until you are 3-4 cm dilated. For some women it takes ages to dilate that much, so you could be having pretty frequent contractions for a while until you can get the epidural. Gas and Air is an easy one, but they may suggest pethidine(sp?), but there are some draw backs to that one. What if you can't get an epidural, what then? Pethidine is the next strongest drug routinely offered I think. Even with the epidural there are different ones available depending on your hospital, basically a low dose one that allows you to remain mobile or a traditional one where you have to stay in bed cause you can't feel your legs.
Afterbirth - I forget the term, but basically do you want to deliver the placenta naturally or have an injection of some drug that speeds things up a bit. I don't know the downsides to the drug, but I am sure you can research it.
Other than that, just make sure that your partner knows what you want and when to ask for things. You should be able to eat and drink during the labour if all is going well, so pack snacks and if you do get an epidural bring some magazines. I found once I was nice and drugged up, I couldn't feel anything so I was just laying there bored until it came time to push. Music options?