It's obviously about more than the birthday. On its own, its a bit thoughtless if he hasn't talked it over with you, just presented it as a fait accompli. However, being away on the actual day isn't really a big deal - as a grown up he/you can celebrate on a different day.
Vegas is definitely more suitable for a boys trip than a family holiday so I can see why, if it's a dream of his to go there, he would go for his 40th and with his mates.
FWIW I went away with DP for my 40th and had a little celebration with the DCs before I went. I don't think they were too bothered that I wasn't around on the arbitrary date, especially as it was midweek and they were at school.
For DP's 40th we went away the week before as that was the only time
we could get someone to look after the DCs. Then for the actual day he went away with his DCs and friends, which we'd discussed. I was welcome to come along too but preferred not to, so happily waved him off. Our celebration was the previous week.
However, we have a respectful and loving relationship and 99% of the time we consider each other and involve each other. It sounds like this is not the case for you, so Vegas with the lads is a symptom of something much bigger.