Although we never had it officially diagnosed, our vet said he believed our dog had developed a mass on her spleen. She had multiple episodes where she would collapse and be virtually unresponsive for hours, and then would recover over the course of a day or two, eventually returning to her "baseline" self. The vet opined that he believed these episodes were caused by a rupture of blood vessels in the tumor, causing internal bleeding. Eventually the bleeding would clot, and she'd begin to recover.

The vet said he didn't think she was in a great deal of pain, and he said he didn't think we needed to make an immediate decision about euthanasia at that time, so we just tried to keep her happy and comfortable. She ended up hanging on for about another year after the first episode occurred, and she seemed to be pretty happy except for when she would have another episode. She finally had a bad one back in January, and she didn't survive. In a way it was a huge blessing that she passed on her own terms and we didn't have to make a very difficult decision.

Hang in there - as you already know, losing a pet is a gut punch. I hope you guys will find peace, and that VNAV will adjust to the new circumstances quickly.