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If she is licking the vaginal area there may also be an infection ( UTI)or bladder stones as this will show bleeding and licking of the area. It would be a good idea to drop off a urine sample to be sure nothing is going on there.
For allergies, which have been particularly bad this year, talk to your Vet about Apoquel or Atopica. The Apoquel will stop the scratching within two two days in most cases.
Sadly when it comes to allergies there is not a lot that can be done to ease the scratching. But you can cut down on the amount of inhaled allergens that may be causing the problem inside by wiping her down with a damp towel after an outing to get the pollen's off of her skin and coat so she is not constantly breathing them in and some find relief in putting an air purifier in the room with the dog to suck out all the dust and pollen's.
If Benedryl is not working very well talk to your vet about using a different type of allergy pill such as zyrtec.
Information ondosing zyrtec http://www.petplace.com/article/drug-library/library/over-the-counter/cetirizine-(zyrtec-)