I recommend checking out Solace. They have been in business for 20 years. It's not open source though but packs all the enterprise features you would need.
That's a shame. I work at Solace so shoot me a message and I will show you how to set it up if you are interested.
Solace's first product was hardware appliances which are still used for high throughput and low latency usecases. Concept of VPN was used to set up isolated virtual brokers so different teams can have their own environments on a shared hardware appliance.
The concept was ported over to software as well and is extremely useful in an enterprise environment. It allows different teams to have their own virtual brokers but not have to pay for or manage multiple brokers.
I still don't get why they use VPN terms for event broker