We did a Lelepa Island Day Tour run by the locals.
The day tour includes transfers from your resort in Port Vila, BBQ lunch, snorkelling gear, morning and afternoon refreshments etc.
It costed us 3600 vatu each but we got a half price deal because we were staying at Pangos Paradise Cove Resort... I'm not sure if that is a common thing or not but it is worth asking your resort just in case.
There are some great caves on the other side of the island..the tour will take you there. THey're really old and there's a very old handprint there..the locals beleive the spirits live there..it's very cool. they lit candles for us and we went in about 70m ..it was great!
The Lelepa Island day tour will take you snorkelling in a few places and they also bring snorkelling gear for you to use if you don't have your own gear...of course, you're welcome to bring your own gear along...it will be very safe the whole time.
We stopped in two different spots for snorkelling and both were great. We spent hours looking around under water. :)
You can do snorkelling basically all around Lelepa (according to the lonely planet) but the tour only took us to a couple of beaches. The first one was great!