"Blood Shield" not draining vim?

[PurpleXVI]

The Demonologist's Blood Shield ability, granted by grafting a Thaureg to the "Offhand"-slot is listed as having "Drain Vim: -2," and as I understand the functionality of sustains with Drain Vim, they should drain that much Vim every turn. That's certainly how Osmosis Shield functions, for instance.

Blood Shield, however, appears to have no Vim Drain actually occurring.

[Marson]

Sometimes a shield will drain only when it is hit (Paladin's Sun Shield). I don't have the talent in front of me to check, but that may be the case here—the shield will drain 2 Vim each time it prevents damage.