Our Constitution doesn't have provisions to deal with a situation like this. There are no redos - there's no Constitutional mechanism in place for them. Assuming Pence is clean (and we don't know that), and the GOP doesn't want to take him down, he'd be next in line if Trump's impeached. The public would view the election as even more illegitimate than it's seen now, but there's no reason to think that'd change anything, except perhaps outcomes in 2018 and 2020 (and maybe not).
Nixon was involved in the Chennault affair where he secretly worked with North Vietnam to prevent a treaty that he thought would have helped McGovern. LBJ knew (but couldn't admit it since he was illegally spying on Nixon), and others had enough to make a really compelling case. Now we have his papers to prove it.
Reagan was by all appearances negotiating with Iran before his election - his known campaign contacts with Iranian leadership were where the Iran-Contra contacts originated, and the hostage release on his inauguration day looks much too perfect. Sadly, Oliver North managed to destroy all the evidence needed to know.
This kind of thing happens. And when it does, we just have to make sure it's remembered, since there's nothing you can do if an election's discovered to be illegitimate after the EC votes.