ΔE 19.6, loose: different enough that you should pick on purpose. Violet Vamp is a little darker, much more muted, warmer than Phthalo Blue (Green Shade).
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Violet Vamp is a little darker, much more muted, warmer than Phthalo Blue (Green Shade).
The same difference applies the other way round: Phthalo Blue (Green Shade) is a little lighter, much 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 Phthalo Blue (Green Shade) on the colour wheel See Violet Vamp on the colour wheel
All matches: Phthalo Blue (Green Shade) equivalents · Violet Vamp equivalents