I'm not much of an Ironscheme user myself, but this is a note to mention
an implementation of functional vectors I wrote, that might be of some
use to you. It is written in portable R6RS and has been tested on Guile
and Vicare Scheme.
The code can be found at https://github.com/ijp/fectors
I will note, however, that the library is not thread safe. It uses
internal mutation to provide efficient access to the most recently used
version of a fector. If this is a problem for you, or you are frequently
using branches other than the most frequent, I also have an
implementation of functional sequences based on fingertrees in my pfds
library, on the fingertrees branch. See
 There are measures I could take to make it so, but I would be
sacrificing some portability, and that is currently my top concern.
"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"