[Shadow] Better VT logic

[mat-simon]

Tentacle Slam, then Vampiric Touch, on undotted targets (10s+ to live, up to 5 dots). Spread adds were getting VT late or never.

• Neutral on stacked ST / 2T / 5T.
• 20k iterations, Archon / Voidweaver:
  • DungeonSlice: +0.3% / +0.5%
  • HecticAddCleave: +0.4% / +0.8%
  • Spread adds: +2.0–4.9% / +3.6–14.1%