First of all we are ready to release Vallum 1.1, the comparison should be done with this one.
Current Vallum 1.0 is very limited while Vallum 1.1 will be much more flexible and powerful. Beta version is currently running on our dev macs and we will release a public beta this week.
I can only make a BIASED comparison between Vallum and its competitors. You should really ask somewhere else
However I'll try to answer.
First of all you forgot to mention RadioSilence, which is a great app, and TCP Block, which is free. Both are application firewalls like Vallum, Little Snitch and Hands Off!. And new competitors are coming too, I bet we will see something after wwdc2016.
Little Snitch and Hands Off! have a different filtering approach, they offer a more granular way to create filtering rules. Little Snitch offers also a powerful network monitor and Hands Off offer options to monitor apps activities and is not limited to networking. Both apps are more complex than Vallum.
Vallum aims at being more easy to use. For example, in Vallum, rules order never matters. This is a trade-off between usability and flexibility.
Then, Vallum offers a default ruleset which is "inspired" by RadioSilence more than Little Snitch or Hands Off!.
Vallum offers interactivity with the PF packet filter allowing to put rules at both application- and network-level, and this is a plus. Then, notification popup alerts are more informative and interactive.
Stay tuned for the 1.1 beta