Minpins are great dogs. Your vet is right. Not only will it be better for her healthwise down the road, but the risk of dying during birthing is much greater in small breed dogs. Often they need medical assistance to have the pups, which can be costly. Why take that chance?
Get her spayed, you won't have to worry about any more heat messes, and she will heal so fast you won't even be able to tell it was done!
Edit: Believe me, she will not be sad. My avatar, Cocoa, was spayed at one year. We waited because she had a horrific case of mange and needed to boost her immune system before putting her under. She is the happiest, friendliest, most playful pin I have ever met. She is so social she has never met a dog who didn't like her. Her favorite thing to do is give people she meets big hugs. There is not a sad bone in her body. Plus I don't have to worry about losing her too early in life from some horrible disease, like cancer. I am confident that I will have her company now until she is at least 16-18 years old.
minpin rescue volunteer