No IDE; I mostly just use Vim and run Arc in a separate window. For highlighting I just tell Vim to treat .arc files as scheme. It works well enough, particularly because I tend to have pretty minimalist highlighting needs. Mostly I just want to see comments and strings distinguished from code (https://news.ycombinator.com/item?id=4478154)
On occasion I find a couple of other tools useful:
What's the benefit of running vim-slime as opposed to just an arc repl from the command line? I've been doing that w/ the same vim/tmux setup you mentioned. I enjoy it.
Edit: I see that you can execute code from the vim window. That is a nice feature. Anything else significant?
No, that's pretty much it. I sometimes find it useful, but yes, mostly I forget it exists. Occasionally it'll bother me when I hit ctrl-c for unrelated reasons :)