It can be great to wear something fresh and new to a party bus at this current point in time, and doing so will undoubtedly give you a fair bit of confidence that you would need to make the most of this experience overall. That said, if your party bus experience will involve you hanging out with friends that you know and love then you might want to think about wearing an old shirt, and in fact the oldest shirt that you own would be the most perfect option for you without a shadow of a doubt.
You see, in order to fully immerse yourself in the Las Vegas NV party bus that you have rented, you need to be able to take advantage of a certain degree of comfort. Without this comfort you might end up feeling somewhat restricted and this could subsequently make you feel like you are not getting a lot out of the money that you ended up spending on this type of thing. An old shirt might not look all that great, but suffice it to say that it would be incredibly comfortable for you to wear.
The starchiness of the new fabric will have mellowed out over time, and this would result in you getting the chance to feel like you are wearing nothing at all. This will free you up and enable you to move as freely as you need to, and on top of all of that you would also get the chance to not feel all that bad if something gets spilled on your shirt. Such a thing is inevitable in a party bus after all so you should prepare for it.