Do you think it is the right thing ? If so, I'll push it to the git. Nothing amazing there, just mzscheme vectors that can be called as hash tables, lists or string. It's got a constructor (vec) and responds to len. Any further function can (should) be written in Arc.
arc> (= v (vec 10))
#10(nil)
arc> (= (v 1) 0)
0
arc> (len v)
10
arc> (v 0)
nil
arc> (v 1)
0
arc> (v 10)
Error: "vector-ref: index 10 out of range [0, 9] for vector: #10(nil 0 nil)"
Edit: Although it might (?) be better in the lib/array.arc, unless of course it's already integrated into ac.scm...
Edit2: Also, I hope (vec ...) is a function, not a macro or special form, so that existing code that uses vec as a local variable for a collection, or as a function, can still work.
It can't be put in a .arc file, since it is pure scheme code. Further functions dealing with vectors should go in lib/array.arc, however. And yes, vec is a function.