•It has fantastic HP and SpA, and good SpA.
•It can boost all of its stats using Z-Celebrate aka Z-disco dance.
•It has access to many good moves such as Wish, Scald, Shadow Ball and Heal Bell.
•It can run both offensive and defensive set.
•Its signature item, Light Ball, that helps it to defeat many NFE Pokemon.
•Has access to Encore.
•Lighting Rod, a useful ability in doubles, although It is not a good Pokemon to use in Doubles.
•Static, that raises the chances of appearing wild electric type Pokemon, a bit useful.
I can't understand why you would use Pikachu, Raichu does it better.
Surely, Vaporeon is the best.
If you are meaning it's 1vs1, then,
Pikachu @Light Ball
EVs: 252 SpA / 252 Spe/4 SpD
Vaporeon @ Splash Plate
EVs: 256 SpA / 252 Spe / 4 SpD
Since Pikachu has more speed, it will move first.
252 SpA Light Ball Pikachu Thunderbolt vs. 0 HP / 4 SpD Vaporeon: 338-402 (84.2 - 100.2%) -- 6.3% chance to OHKO
252 SpA Light Ball Pikachu Thunder vs. 0 HP / 4 SpD Vaporeon: 416-492 (103.7 - 122.6%) -- guaranteed OHKO
252 SpA Splash Plate Vaporeon Scald vs. 0 HP / 4 SpD Pikachu: 240-283 (113.7 - 134.1%) -- guaranteed OHKO
252 SpA Splash Plate Vaporeon Hydro Pump vs. 0 HP / 4 SpD Pikachu: 331-390 (156.8 - 184.8%) -- guaranteed OHKO
But if Vaporeon moves firsts for Choice then,
252 SpA Vaporeon Scald vs. 0 HP / 4 SpD Pikachu: 201-237 (95.2 - 112.3%) -- 68.8% chance to OHKO
252 SpA Vaporeon Hydro Pump vs. 0 HP / 4 SpD Pikachu: 276-325 (130.8 - 154%) -- guaranteed OHKO
This means if Pikachu moves first and use Thunder, it will win. And if Vaporeon moves first and use Hydro Pump, then Vaporeon will win.
But if Pikachu use Thunderbolt and Vaporeon use Scald, then Vaporeon wins.
Hope this helps.