ΔE 13.7, fair: the same colour family, but not a straight swap. Vampire Fang is much more saturated than Longbeard Grey.
Can I use Vampire Fang instead of Longbeard Grey?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Vampire Fang is much more saturated than Longbeard Grey.
Can I use Longbeard Grey instead of Vampire Fang?
The same difference applies the other way round: Longbeard Grey is much more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.