Talk about a 4MSS: Hydration Vaporeon gets Shell Smash and Tail Glow, we know that much yep. It gets Hydro Pump/Surf and Ice Beam naturally, which is a good start but now comes the coverage: Vaporeon has access to Thunder/Thunderbolt, Hurricane, Stored Power (140 BP after 1 SS, fun thing is that Vaporeon learns it naturally) and a dozen other (less useful/powerful) coverage options. Keep in mind that Vaporeon also appreciates Rest in his moveset and you just wished you had 6 moveslots (or more).
All vaps needs is grass knot, either SS or tail glow, surf and rest, want some proof, here's some calcs:
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 0 SpD Eviolite Chansey in rain: 45.17 - 53.26%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 224+ SpD Celebi in rain: 45.29 - 53.46%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 252+ SpD Dragonite in rain: 46.37 - 54.66%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 136+ SpD Kyurem-B in rain: 46.91 - 55.5%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 228+ SpD Amoonguss in rain: 48.61 - 57.4%
+2 252 SpA Splash Plate Vaporeon Surf vs. 248 HP / 216+ SpD Empoleon in rain: 49.32 - 58.22%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 244+ SpD Venusaur in rain: 49.45 - 58.24%
+2 252 SpA Vaporeon Grass Knot (80 BP) vs. 0 HP / 4 SpD Ludicolo: 51.16 - 60.46%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 0 SpD Virizion in rain: 51.55 - 60.62%
+2 252 SpA Splash Plate Vaporeon Surf vs. 252 HP / 208 SpD Ferrothorn in rain: 51.98 - 61.36%
vaps should honestly always run shell smash over tail glow because it's already breaking the walls it needs to while nuking this offensive metagame
really short argument, I just wanted to show these calcs