Vaginal discharge can be a sign of pyometra this is a uterus infection that gets bad very quickly it's very dangerous and fatal without treatment. It happens when the dog has been in season within the last few months other symptoms can include excess thirst, depression, lack of appetite. This happens when the dog hasn't been spayed.
First, are you sure it is vaginal? An older dog (or cat) can have an anal mucous discharge that can be "colonitis". It doesn't smell like feces, and is clearish and colored. It is an infection that antibiotics can take care of. If you are sure that it is vaginal, I would not hesititate to get the dog to the vet. It could also be an infection that could be treated with antibiotics. Dogs can get yeast infections just like humans do. But it could be something we don't have any idea of, and the vet would be able to test the discharge and then you would know if they can treat it with medicine, or whatever else your doctor suggests.