Nuln Oil vs Vicious Purple

How close are they?

ΔE 54.8, loose: different enough that you should pick on purpose. Vicious Purple is noticeably lighter, much more saturated than Nuln Oil.

Can I use Vicious Purple instead of Nuln Oil?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 55). Vicious Purple is noticeably lighter, much more saturated than Nuln Oil.

Can I use Nuln Oil instead of Vicious Purple?

The same difference applies the other way round: Nuln Oil is noticeably darker, 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.