If you're making the slaw more than an hour in advance, keep it in the refrigerator; let it stand at room temperature for about 30 minutes before serving.
Toss together bok choy, vinegar, both oils, and chiles in a large bowl. Season with salt. Let stand until bok choy collapses to half its volume (it will darken slightly), at least 20 minutes. Toss again before serving.