I was wondering why your console output never shows a path with /vendor in it because your recursive logic intends to create a path that has /vendor in it. Then, I realized, that's because this logic:

node = node ? __dirname : node;

is backwards and will always select __dirname when you pass a value for node. So, every recursive call ends up using the same value for node and thus an infinite loop that eventually overflows the stack.

Instead, you want this:

node = node ? node : __dirname;

The MDN description for the ternary operator is this:

condition ? exprIfTrue : exprIfFalse

Another way to do this is to use default parameters and let the JS engine supply a value when nothing is passed and write is this way:

const packageComposer = async (node = __dirname) => {
   ...
};

For any further debugging, I might suggest this cleaned up version with logging that lets you follow where it goes:

function packageComposer(node = __dirname) {
    // read and parse composer.json using require()
    const filename = path.join(node, 'composer.json');
    console.log(`About to read ${filename}`);
    let composer = require(filename);
    const dependencies = Object.keys(composer.require);
    console.log("Found dependencies", dependencies);
    for (const key of dependencies) {
        const pieces = key.split("/");
        const dir = path.join(node, 'vendor', dependency[0], dependency[1]);
        if (fs.existsSync(dir)) {
            console.log(`Calling packageComposer(${dir}) recursively`);
            packageComposer(dir);
        }
    }
}

As I said in my comments, a couple things seem odd here. You do key.split("/") to get the pieces of a key such as "alek13/slack", then you immediately put them back together again to reconstruct "alek13/slack" again. Not sure why you're doing that or what you expect to happen when the keys can't be split and you've got a key like "php" that doesn't contain a "/".