Been there and done that.
One way or the other it will come out...but your dad will most likely DENY the whole thing....LOL ...and blame you for "misinterpreting"... as did my father..
The nature of cheating is deceit and lies. Layers of it.
Nothing worse than being the last one to find out....your mom will be hurt like hell. It may even be the last straw for her for all you know...
....were it me...I'd want to know...Living a lie is worse.
You may want to drop the bomb on your dad Pre-Vacation....it will give him the time to come clean with mom....and her the time without family around to let him have it full blast.