He mentions it in his book “promises to keep” published in 2008. You can get an ebook/book from your library if you want to verify that for free. He also spoke at the 2004 National Stuttering Association conference about his stutter - that was before he was VP. Here’s a video from 1994: https://www.c-span.org/video/?c4841336/user-clip-joe-biden-stutter-stuttering