The vaporeon mirror match is actually really easy. You just need to get the dodging rhythm down. Open combat with two dodges to the quick attacks they get right after it says go. Get one attack in then another dodge. After this you settle into a rhythm of three attacks and a dodge or two attacks and a dodge if you've got a spot of lag. The slow moves they make are super easy to dodge with this because there's an extra delay while you're waiting for the flash to appear so it's clear what's going on. If you're feeling risky you can work one extra quick attack in before their strong. Follow their strong with one of your own or with 5 quick attacks if yours isn't up at the time. Return to the normal 2-3 quick attack dodge rhythm rinse and repeat.

If their slow attack is a quick charging one they may have two built up, depending on your charge move you may need to let them blow two of them before you can safely sneak yours into the mix.

It takes a bit of practice to get the rhythm down but once you do the mirror is super easy on vaporeon and given his dominance in the meta that's a skill worth picking up.