Vampire Red vs Dark Crimson

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Dark Crimson is noticeably darker, more muted than Vampire Red.

Can I use Dark Crimson instead of Vampire Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dark Crimson is noticeably darker, more muted than Vampire Red.

Can I use Vampire Red instead of Dark Crimson?

The same difference applies the other way round: Vampire Red is noticeably lighter, more saturated.

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.