It's more than just sharing code. It's all about your routes. In an isomorphic web framework: You write a single description of a route handler as a function from JSON to VDOM. That handler will load different dependencies on the client/server to create DOM/HTML. This is where the isomorphism thing comes from: the HTML generated on the server, once loaded by the clients HTML parser, will be equivalent to the DOM that would have been generated on the client. You could of course do this normally but you'd be responsible for maintaining a server operating on HTML in coordination with a client operating on DOM. An isomorphic web framework unifies these two worlds.
An isomorphic framework itself is effectively a proof (in the mathematical sense) of the statement: client-side rendering and server-side rendering are isomorphic.
That still seems like an overly pseudo-mathy way of describing that you render some data structure (possibly just a string, in the case of Rendr), and then the framework produces either HTML or DOM operations. Regardless, to my knowledge, it's not necessarily the case that isomorphic frameworks produce VDOM, rather than HTML on both sides. If I'm reading their site right, Meteor only seems to have gained VDOM-like functionality with the release of Blaze less than a year ago.
Totally agree on VDOM, it's an implementation detail.
I also agree that isomorphic is an intimidating and esoteric term. There are examples however, like the word function, which are "overly psuedo-mathy" in exactly the same way. The mapping between rendering technologies is basically an isomorphism. Each category has objects (html OR dom) and a we can map them with the html parser (html => dom). Each category has morphisms (json => html OR json => dom) and we can map them between by loading the client modules ((json => html) => (json => dom)). Finally, for some json input (J), a renderer (R) and a function (F) that performs those mappings, the framework ensures that: F(R(J)) === F(R)(J). That's not exactly an isomorphism but it's VERY similar.
An isomorphic framework itself is effectively a proof (in the mathematical sense) of the statement: client-side rendering and server-side rendering are isomorphic.