Unfortunately, that vet has no idea what he/she is talking about. They don't require a bowl to soak in, as long as you keep humidity within the proper range. Make sure you measure humidity with a digital hygrometer, and not those cheap circle kinds that are inaccurate.
If a BP who doesn't normally soak is soaking, generally it means it is either too hot, too dry, or it is infested with mites.
It's not "bad" of course to provide them with a large bowl, but it's definitely not a requirement for a healthy snake.