SLIME is, but there is Slimv which is a SWANK client for vim:

https://github.com/kovisoft/slimv

(Never used it myself so can't comment on it beyond noting its existence.)

I have an article I wrote about how I did this, but may have lost it to time.

The short version is I have SLIME running inside Emacs, and my text files open in vim. I would just reload the changed file(s) when needed (originally with cl:load-file, then later with asdf, which is roughly "make" for lisp). I used slime similarly to how I would use gdb. It was a big improvement over using vim with the clisp[1] REPL both because SLIME was more powerful than the clisp repl, and because it worked with other implementations like CMU CL (which sbcl now is a fork of).

I mostly just used the menus for interacting with emacs, but since the shortcuts are listed beside commands, I did learn the more commonly used ones.

I continued like this for years until I discoveredevil-mode[2], which was the first thing close enough to vim to be usable for me (and even then I had to add a few keybindings that it was missing).

1: clisp is a specific implementation of common lisp, not an abbreviation

2: https://github.com/emacs-evil/evil

To provide a bit more context, most of SLIME is just Common Lisp code (https://github.com/slime/slime), with a bunch of Emacs Lisp code alongside to support interfacing with Emacs. But you don't need that Emacs Lisp code to take advantage of almost all of the functionality SLIME provides. For instance, if you want to know who-calls a function, there's some command in emacs to do it, but all that command is doing is just a bit of elisp code which sends a message to Swank (a server running inside Common Lisp) and Swank invokes some native CL code to figure that out and return the results, then finally a bit of elisp code presents the results in some way. Vim can do the same thing just fine with vimscript/python (what the Slimv plugin uses) or otherwise, the bulk of the work in figuring out the list of callers of some function is done by the CL code (and CL implementation itself).

With this: https://github.com/jpalardy/vim-slime

It doesn't do everything, it doesn't do it as elegantly, but it works

You don't. You run it in emacs. That's what they are saying.