I can only speak from my own personal experience and the info I got from my therapist, but most vaginismus patients (not all of them) have some issues with sex in general, which could mean they either don’t masturbate, can’t have an orgasm, and/or never explored their vulva and vagina before.
During my second or third session this subject came up and my therapist told me dilation is much easier when you’re aroused, since you tend to be more relaxed, there is more wetness and the vagina opens up slightly. She never told me it was a deal breaker, but she did give me the advice to try it out.
As a student I masturbated once in a while and I knew how to get clitoral orgasms, so I looked up my favorite book again in the library and read some of my ‘favorite’ (you know what I mean) paragraphs before starting with the exercises. I’m sure that did help.
As you already wrote, an orgasm does the opposite: you get dryer, the vaginal walls contract and the vaginal opening gets smaller again. All those things are to be avoided when you’re dilating, so make sure you’re aroused, but not to the point an orgasm is imminent. That still counts these days when I’m having sex with my husband. He needs to come first. I’m not sure if this is something other patients recognize, but after my orgasm, penetration is A LOT harder.