I'm going to Vegas in two weeks! I was also there in April... I love Vegas!
Daytime wear is pretty much what you'd wear any other day. If you're shopping, eating or just wandering, wear a cute top and maybe shorts/a skirt, or a sundress. You'll see some people are dressed up 24/7, but you'll also see a lot of tourists dressed super casual too.
As everyone else has pointed out, bring a swimsuit! Or several! And ditto what's been said about the weather - it's SUPER hot outside, but indoors the AC is always on full blast, and it does get chilly. If you can, carry a light jacket/cardigan with you at all times.
As for club/nighttime wear - I've done simple mini-dresses, cocktail dresses, and even a halter and shorts. The only real rule of thumb is to look hot
Advice: get yourself on the guestlist for clubs. Google the clubs you're interested in, and you can usually e-mail promoters who will add you. You can also call the club 24 hours in advance, and they should be able to add you as well. If you're with all girls, it's usually very easy to cut the line even if you don't call ahead. The guestlist does help though - I was in Vegas with 3 friends (2 guys, 2 girls including me) and we were on the guestlist for Tryst at the Wynn, (awesome place btw) and we literally walked up to the bouncer and walked in right past an entire line of people.