Just wanted to chime in to reiterate: There is no such thing as giving too much information to the people who you are relying on to show you a good time! Especially at a place like V&A; they ask you all sorts of questions when you make the res, and then they call you again before your arrival! These people want to know what they can do to make you happy! They routinely get my favorite bottle of wine (which, oddly, they don't have on their darn near encyclopedic in-house list) via special order so we can enjoy it with dinner. Additionally, they're great about food allergies and even preferences. I'm a vegetarian and I always get taken care of phenomenally there and my friends I usually go with are opposed to fish, among their many other quirks. Again, I can't emphasize this enough, the restaurant will only know if you tell them!!! They're great, but they're not mind readers!
Specific benefit of arranging it in advance: you can have WDW's cake service make a specialty cake for after dinner! All the restaurants keep the cruddy little $10 cakes on hand for people who don't plan in advance, but Disney provides a better option for those who are into planning!
As far as attire: I've certainly never seen a lady in a sun dress in there, but who am I to judge? We always do ties (in addition the required jacket) for the guys and an evening gown for our lady friend. (Of course, a tie is a minimal concession in exchange for the lovely company the lady...)
BTW: It doesn't hurt to mention what's going on (a b-day, anniversary etc.) when making every single reservation for your trip! They'll note it in the computers so people can fawn and coo over you everywhere you go! Also, they'll do whatever they can to make it even more special. (Example, we have been upgraded, for free, to a suite as well as being invited into the AP soft opening of Monsters Inc Laugh Floor and other things too all as a result of birthdays in the parks! (Don't expect or demand these things, of course, but it's really nice when it happens, and most CM's really enjoy doing these things for people just for the smiles and the thank yous!) And be sure to make your birthday-person wear their B-Day badge proudly (we've been told you can even wear it when it's not your birthday, it just has to be during the week of...)! They'll probably forget they have it on and just start having fun with the fact that their whole 'disney family' knows it's their birthday!
Have a great time and enjoy V&A (like you needed me to tell you to have a good time! Of course you'll have a good time, it's V&A, you can't help it!)