Vivid Volt vs Valkyrie Yellow

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Valkyrie Yellow is much more muted, warmer than Vivid Volt.

Can I use Valkyrie Yellow instead of Vivid Volt?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Valkyrie Yellow is much more muted, warmer than Vivid Volt.

Can I use Vivid Volt instead of Valkyrie Yellow?

The same difference applies the other way round: Vivid Volt is 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.