If you were vague when confessing a mortal sin, but didn’t intentionally leave anything out, do you have to reconfess it?
Also, I lied to my dad about being on a lustful site once to stay out of trouble, do I have to show him it now?
you would not have to show your father the site or tell him you lied, because it is not something that has hurt him or caused any damage that can be repaired,
the only one you need to seek forgiveness from in this case is God, and the one that needs repairing from the sin is you,
as for re-confessing a mortal sin you were vague with,
if what you left out changes the nature of the sin, such as whether or not it involved other people, or if others witnessed you sin, or how many times you committed it, or whether the sin you committed had to do with someone married or consecrated to God(priests or religious),
things like that change the nature of the sin, and can make it more or less severe, and so if you have omitted some important detail such as that, then you would need to confess again.
and when in doubt, just ask your priest, he will probably be able to give you better answers to your questions.
ok, hope this helps, take care.