That's a seriously awful vacation policy - even for a surgery center.
12 people & 8 weeks of summer means some people are never going to get a summer vacation - especially when you add in the other things you have to schedule around (school schedules, camp schedules, holidays, family commitments, etc.)
What if you were further down the line, & someone else picked the 2nd week of August? Then you wouldn't get a summer vacation anyway.
Have you spoken to the co-workers that are after you? Do you know for sure if you don't take the 2nd week of August someone else will before it gets to the Mother of the Bride?
You're in the proverbial "stuck between a rock & a hard place" situation - thanks to both the surgery center's policy & the Mother of the Bride letting everyone know she needs that week.
I do think family weddings trump summer vacations, but, that said, I'm not sure what I would do.
Like others have said, I wouldn't want to be "that person" especially if everyone else has purposefully left that week open. However, I also wouldn't want to miss a chance for a summer vacation w/ my family before my son goes to college either. And, admittedly, if my co-worker were the type to not "help out" & cover for others as needed, it'd be more difficult for me to give up the week - but, again, I'd have a hard time taking the week.
June is half a year away still. Have you even looked at the possibility of rescheduling your son's graduation party? Would it be possible to switch your vacation & the graduation party & turn the graduation party into a "Congratulations on your Graduation/Going to College!" Party in August instead?