Success! Sob and cdiedrich, thanks for your responses. I do have a third RB750G so I've configured it as "router C" between A and B to act as simulated internet. My expectation that the IPsec connection would add its own routing, or that a route for the far side of the IPsec is unnecessary...

I'm working on a setup that will eventually turn into a site-to-site VPN via the internet. For now it's a collection of "test lab" hardware all on one desk for easy testing. The topology is as shown in the attached image: one laptop connected to router A using the 10.0.84.0/24 network, one...