I just did a quick look-up and it looks like a vulvoplasty is the treatment most vets recommend. However, it would appear that it's not effective if a dog also has vestibulovaginal stenosis
.
http://www.ncbi.nlm.nih.gov/pubmed/11804321?ordinalpos=1&itool=EntrezSystem2.PEntrez. Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.P ubmed_Discovery_RA&linkpos=2&log$=relatedarticles& logdbfrom=pubmed
http://www.uoguelph.ca/~rfoster/repr..._dog_vulva.htm