i wouldn't have her spayed while she's in heat, personally. usually it's recommended that they wait 6 weeks after the end of their heat. if that's a problem with the voucher, it may be possible to have your vet contact the shelter and explain the situation and their professional opinion. i know the one place i work we had an EXTREMELY obese middle-aged lab come in and was supposed to be spayed by x date, but the vet called the shelter and was able to get an extension for them in order to give the dog time to lose some weight.