ΔE 12.7, fair: the same colour family, but not a straight swap. Violet Vamp is a little lighter, more muted, warmer than Dark Ultramarine.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Violet Vamp is a little lighter, more muted, warmer than Dark Ultramarine.
The same difference applies the other way round: Dark Ultramarine is a little darker, more saturated, cooler.
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.
See Dark Ultramarine on the colour wheel See Violet Vamp on the colour wheel
All matches: Dark Ultramarine equivalents · Violet Vamp equivalents