Ok, I actually think Vaporeon is more viable than Volcanion due to Volcanion lacking a way to boost his SpA. And Vaporeon deals more damage than Volcanion after just 2 Calm Minds when the minimum you want is 3. Heck, you can get up 4 Calm Minds and destroy the shield before Blastoise resets you.
+2 252+ SpA Tera Fire Vaporeon Weather Ball (100 BP Fire) vs. 0 HP / 0- SpD Blastoise in Sun: 255-301 (85.2 - 100.6%) -- 6.3% chance to OHKO
252+ SpA Tera Fire Volcanion Weather Ball (100 BP Fire) vs. 0 HP / 0- SpD Blastoise in Sun: 192-226 (64.2 - 75.5%) -- guaranteed 2HKO
+2 252+ SpA Tera Fire Vaporeon Weather Ball (100 BP Fire) vs. 0 HP / 0- SpD Blastoise in Sun: 255-301 (85.2 - 100.6%) -- 6.3% chance to OHKO
252+ SpA Tera Fire Volcanion Weather Ball (100 BP Fire) vs. 0 HP / 0- SpD Blastoise in Sun: 192-226 (64.2 - 75.5%) -- guaranteed 2HKO