Just based upon your title, I'll take a random stab at this.
I'm assuming that by "VISTA" participation, you are referring to being a VISTA/AmeriCorps volunteer? Having been one in the past, I know that they don't pay you a salary per se, but they do pay you a stipend.
The rules for SSDI are clear. ANY income you earn that can be used towards food and housing is considered income for the purpose of considering your SSDI eligibility. That means whatever stipend you earn is considered income, and counts towards your income limits. If the VISTA stipend puts you over the eligibility limit, then you may end up losing your benefits.
Of course, you are welcome to supplement with more information to help us answer your question.