So I asked my friends to go to Vegas for my birthday, am I expected to pay for all of their rooms? It would be around 3 rooms I would have to pay for. I am the one planning it and I did invite them.
There doesn't appear to be a strong consensus on who is expected to pay for something as big as a vacation, only that the host must be clear about what they are asking guests to pay for up front - standard etiquette for any sort of event, really!
From the New York Times, the president of a travel consultancy that plans destination celebrations advises:
BE UPFRONT ABOUT WHO PAYS Some hosts cover the cost of everything for their guests, including airfare, accommodations and meals not part of celebration events. Others spring only for events associated with their celebration. When you’re inviting guests, clearly convey which costs you’re picking up. “Money is a sensitive matter,” Mr. Ezon said, “and I’ve seen instances where some guests assume that every part of the trip is covered but get socked with an unexpected hotel bill. A scenario like that can hurt your valuable relationships.”
From a publication geared towards a slightly younger crowd, Refinery29 quotes etiquette expert Myka Meier (focusing their article mainly on the opposite problem, of friends getting many such invites):
“It is presumptuous and can be considered rude to plan an event with the thought that your friends will chip in. I recommend planning a celebration that you can afford so that you don’t put your guests out of pocket.”
To be clear, there’s nothing wrong with having a BDB ["Big Deal Birthday"] for birthdays that are actually, you know, a big deal. If you’re turning 25, 30, or 40, for example, those are major milestones that are worth celebrating, and the people who care about you will probably want to help you do so.
As long as you're being relatively chill about it, there's an argument to be made for leaning into a little birthday extraness while you can, before kids and work schedules and all the other boring trappings of adult life begin to get in the way. But as with bachelorette parties and other events, it’s crucial to be respectful of varying financial situations. Just because you can afford to spend three figures on dinner doesn’t mean everyone in your social circle can (or wants to). So unless you’re in a position to pay for everybody or are okay with certain people not being able to swing it, maybe pick a spot or activity with a more reasonable price tag.
I think an important point to call out here is that Meier talks about planning an event assuming that friends will chip in. It might seem subtle, but that's a bit different in my estimation than planning an event that you'd hold regardless and asking people to split - think "I would like you all to pay for a night in the penthouse suite" vs. "we could save some money if we all stay in this Airbnb". (The former is something I'd only expect for a very special event like a wedding party and requested by someone who was not the main beneficiary (e.g. maid of honor); the latter is something that I've encountered a few times with friends, as college students or young professionals in the US, which everyone involved thought was a reasonable and even smart idea.)
Finally, I want to point out that the advice for a birthday vacation differs from smaller, less-costly events like a dinner, which etiquette experts agree ought to be paid for by the host if possible. As mentioned in that article, etiquette is situational: what is "OK" to ask your friends to pay will depend on your (plural) circumstances. I don't think most rich celebrities would ask their friends to chip in, but for us regular folk it's common enough that you can find advice on personal finance blogs about how best to budget and track who owes what on such a trip. Again, the only constant is be upfront.
To sum up - if you can afford it, it would certainly be nice of you to offer to pay for the rooms. But as the saying goes, it's an invitation, not a summons - you're free to ask them to split the bill, and they're free to decline. Just be clear and take any "no" RSVPs gracefully!