|I think in general no matter what you're soaking in, the hotter the water the more effective the soaking agent will be. Baking soda is sure worth a shot, as it's not going to be tough on your diaper.
There are always the big guns, like a bleach solution, but some things I'd think of trying first:
-Hang it outside on a sunny day. Or two. UV rays are amazing things.
-Try another strong vinegar soak in hot water (maybe if you want to follow the baking soda soak?). Or how about mostly just hot vinegar? Again, unlikely to hurt anything except the smell.
-Soak it in hot water with quite a bit of tea tree oil. It has some anti-bac, anti-fungal properties, and may help while not hurting.
-Try something like BacOut. I have their carpet cleaner and have been amazed at how well it gets out even barf smell.