Clinton and Pence both hired a law firm to determine which emails would be considered private and which emails would be subject to the records keeping act. It was not illegal for neither Pence nor Clinton to use a private (non-gov) account, as long as they submitted all "official business" emails for record-keeping. Both did.
There is no material difference between using an AOL account or using a private server. Indeed, one could argue that using a private server you can at least account for who have had access to the emails. In the AOL case, there is no way of knowing. A private account - on AOL or a private server - cannot be used for classified material.
In the Clinton case it *was* determined that she had sent
- some emails where the contents was retroactively classified. This is not criminal, as Clinton the material *was not* classified at the time.
- A total of 3 emails which contained classified information at the time. However, the "classfied" markings were non-standard which could explain why Clinton did not notice them.
It was not illegal to set up at private server. Clinton was clearly aware that she should not use it for classified material; otherwise you would see a lot of classified material with standard markings on the server. Which there was not.
Maybe she should have realized that there was a risk that she may accidentally send classified material. IMO the greater risk was that state dept. employees would send classified material *to* her account. Was it reckless? Possibly. Criminal? No.
If Pence has sent classified material from his AOL account, it is equally illegal, regardless of whether the account was "official". If he did not instruct aides to avoid sending classified material *to* his account, it would be equally reckless.
Fun fact: Pence was hacked. Clintons email server was not.