According to Marvel lore, vibranium is the most powerful, most durable substance known to man. However, even though Mark suits are very resilient, Tony Stark never used vibranium to make them. One of the main issues with vibranium is that it's very rare, making it extremely expensive. Although Tony Stark had personal relationships with the Wakandan people and could easily get ahold of the substance, it didn’t make financial sense. In other words, he would have to use this precious metal all the time, thus wasting his personal resources and the limited natural resource.