I was able to get around this. Each time I got that far in the Prism Vault, I found two prisms with a beam between them that I don't think should have been there --- it didn't come from the "main" beam, and when I redirected either prism the beam disappeared and didn't return even when the prism was returned to its original position. One of those prisms was north of the one that had been causing the crashes, so I went up there first and turned the prism, making the beam disappear, then went back and turned the crash prism, which then worked great, and I finished the level. Can't wait to see the rest of the game when it's done.