The expression parsing is split into a couple distinct steps, the first is tokenization, where we parse the string and segment it into things like literals (constant numbers, strings, etc.), operators (+,*,etc.), and variables. I believe this step already parses out and segments string literals (things like: "There were "), so that half is done for you. The other half that is left to do is turning those tokens into blueprint nodes and connecting them.
Right now, you’d have to do something like: “vec(x,y,z)+vec(1,2,3)+vec(u,v,0)”. You could change the var type after the node was generated, but you’d probably have to replace the nodes that they’re plugging into as well (to be vector specific functions).