Ask to share expenses for a trip to Vegas??!?

Yes. It would be rude. Also, why have you never thrown him a little celebration? You could have cake and friends over. There's no excuse for him never having had a party before.

I wouldn't ask for people to help pay for him going to Vegas. You should just have a dinner party but if people want to bring a gift and ask for suggestions you can say a cash gift would be appreciated to help him get his trip to Vegas. With the cheap flights from most places these days you could probably skip the party altogether and use the money you would have spent on that to pay for his trip. Check out Expedia, etc. for the cost and you may find some great deals. If not, just have a dinner party to celebrate but don't ask people to help pay for his trip.

Are you asking for people to contribute to HIS trip or to go with you guys for a weekend celebration?

Usually when people give a gift they get something out of it kwim? i.e. the party. Maybe throw a dinner party that you pay for and if people ask what they can give him say the only thing he really wants is a trip to Vegas and that they can either (a) bring their own gift that they choose or (b) contribute towards a trip for him.

Yes. It's very rude to expect your family and friends to cough up the money for HIS vacation. If you can't afford a trip to Vegas by saving, then throw a dinner party, but don't go asking your friends and family. What are they...vacation welfare, now?

You could let them know about the trip and if they CHOOSE to give him money towards going that's one thing, but shilling for money from your friends is really tacky.

I don't think it's rude at all. I think many people would welcome not having to figure out what to get him, shop for it, and wrap it. But make it an option to participate and figure out a reasonable and set amount to donate. That way no one will feel obligated to join if they can't afford the amount and those that do participate are all giving equally. Some people don't like it when they give $100 towards a group gift while another only gives $40 and gets the same amount of "recognition".