Sorry don't know about the airmiles, but we have just come back from Coconut Bay Beach Resort in St Lucia and I would thoroughly recommend it - there are loads of reviews on tripadvisor. It worked out a lot cheaper booking the flight separately through Virgin and accommodation through Expedia.
We had the same issue a few years ago and ended up having a fab time in St Lucia - lovely beach, very friendly, we hired a car and drove round. We actually stayed in a large family suite in the coco palm resort, but I think I booked that separately to the flights.
Thanks to work have virgin air miles, looking to use for a family holiday in the Caribbean (2 adults, DD 4, DS 18 months. Is it most cost effective to use air miles for flights and book separate accom or to take the Virgin holidays discount? Also, any rec's for family friendly Villas in the Caribbean?