Instead of sending the letter first, I would mention that your parents 50th anniversary is coming up and you really want to do something special like send them on a vacation or something. Ask your siblings if they have any suggestions. It could be that they have already thought about it and might have some ideas of their own. If not, start dropping hints about Hawaii. Maybe ask them to do some reseach as far as which island, hotels, etc. It could be that they will be more open to the idea if they think they had a part in the decision instead of you deciding what they are going to do and how to do it. Also, if they are concerned about the cost, maybe a 7-day trip is more reasonable. Lastly, I think it would be tacky to put the money in separate envelopes. You are giving the trip as a combined gift so it should be place in one envelope.
However it turns out, it is a lovely gesture on your part and I'm sure your parents are going to be thrilled.