Indeed Twin Spike Heroic appears to be far superior to HP normal.
As for Vigor, first make sure you are doing it right. Whenever you change total energy, you need to change "Do not use if energy is above (HaT only)" setting in the DPS rules. It's there to prevent energy capping in GCD-starved situations, but in this case, you need to change it from 100 to your total energy (110 if you have vigor, 120 if you have it glyphed too). I am seeing the following regarding Vigor:
- dps with Heartpierce increases by 70 when you have glyphed Vigor
- dps with Twin Spike increases by 30 when you have glyphed Vigor
The effect if Vigor on dps is higher for Heartpierce because you have higher energy regen and therefore higher chance to get energy capped. Vigor+glyph reduces the chance to get energy capped. This may also indicate that reducing or eliminating energy pooling may be beneficial to your dps. Check "Do not use if energy is below (energy pooling)" setting for envenom. By default it is set to 60.
P.S. I was right about energy pooling. Consider this table:
Energy Pooling Vigor+Glyph Heartpierce Twin Spike
Yes No 12998 13148
Yes Yes 13069 13181
No No 13021 13153
No Yes 13047 13181
As you can see, the effect of vigor is seriously diminished when you do not have energy pooling. Also energy pooling is definitely limiting the effectiveness of Heartpierce. Also energy pooling is still effective for Heartpierce, but only when you have more energy. It's probably worth also trying an intermediate energy pooling case, say 50 energy.