My husband and I feel are have failed to keep her safe.  We know we haven't done anything to hurt her only to help but you still feel like you've let her down. 

I agree, what a lucky boy to have humans who care so much! It is absolutely not your fault. VAS seems to be generally considered rare by vets (although being here, it doesn't seem so) and is not something I had even heard of before October. Everything you have done in the past few weeks, as well as the amputation, is take the best possible steps TO keep him safe and to give him a chance at continuing his happy, loved, life. 

There is no blame to be laid. It's just the brutal nature of life that loving something dearly puts it in a position to hurt us deeply and we wish we had control over these things...bur we don't. They just happen, sometimes against all the best odds. The pain and worry doesn't make the love and joy less worthwhile for yourself or your little one. And before you know it, the painful and worrisome part will be over for all of you. 

Hang in there. We're all thinking of you!