spays absolutely can be done while in heat. while the blood vessels are larger, they are also larger once a dog has gone through one heat, period - a spay is never as easy as prior to the first heat! there is a risk of bleeding out from ANY surgery - you don't need to be in heat for that to be a problem. good surgical technique can limit the risks, in heat or not.
talk to your vet- they may board her, but it will be with much more strict instructions. i have always boarded intact females in heat, but there is an additional fee since the bitch needs to be kept away from EVERY other dog and we do not board them in the general wards.
and i agree - i encourage everyone to spay BEFORE the first heat for 99% of folks out there.