Pros
Working at Vaco is marginally better than a series of one-year contracts. If your team is eliminated Vaco will have you apply for other contract roles they're providing staffing for. It works a bit like a contract-only job search. You get to work at their client's offices which are often nice in my area and your full time coworkers can often be great. My VACO manager was a decent manager and seemed happy to let the client company manage me. My fellow Vaco contract workers were easy to work with and the office culture was respectful.
Cons
VACO abruptly lowered our benefits (which were already low) to below my original offer letter. Holiday pay was affected and the client company wasn't open on the days we were no longer being paid for. The only way to be paid for that time was to work at normal pay rates during the holiday. This effectively reduced our pay by several days worth of hours. While I was there I often felt upper management was trying to pressure us into not asking for raises, benefits, or vacation time. It seemed like a system set up to keep us as permanent low-benefit employees with little opportunity for advancement.