Ok for everyone who want's to maximize their vessels Damage here is a quick roundup of how the Vessels Damage is affected by all the modifiers you got out there.
There is the Base Damage of the Vessel (BD) that increases with skill level and char level. How the progression looks like I cannot tell, but with higher levels I assume that the base damage has diminishing returns.
Now we take the BD and the number displayed as our Vessel Damage (VD) is calculated the following way:
VD=BD*(1+SD/100%)*(1+syn/100%)
.
Where:
SD = phys Spell Damage% + Pasive Bonus from Life and Death/Solstice and Equinox + xx% Bonus to Vessel Damage from items.
syn = the number displayed on the bottom on the skill description of the Vessel Energy: xx% increased Damage
Look up how this Synergy is calculated
hereSo stacking Spell damage is much less rewarding than investing in Energy and Energy factor at some point in late game.
For example: with Energy = 4000 and EF=750 the factor Synergy has reached it's cap of 200% and you will get syn=445%.
Since Paladins have no other skills to increase this synergy Percentage you should try to reach the Factor cap while not sacrificing too much Spell Damage.
Also note that Passive Bonus and Spell Damage only stack. the Passives give 7% damage Bonus per skill level. so 1 skill > 7 phys Spell Damage since it also increases the Base Damage of the Vessel.
With high Values of Energy (>3000) a plain +15 Energy factor to Spell damage jewel is better than a Hel rune as long as the EF Synergy is not capped already.