Dark Magenta vs Vermilion

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Vermilion is a little darker, more saturated, warmer than Dark Magenta.

Can I use Vermilion instead of Dark Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Vermilion is a little darker, more saturated, warmer than Dark Magenta.

Can I use Dark Magenta instead of Vermilion?

The same difference applies the other way round: Dark Magenta is a little lighter, more muted, 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.