ΔE 19.7, loose: different enough that you should pick on purpose. Vermilion is a little darker, much more muted than Flat Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Vermilion is a little darker, much more muted than Flat Red.
The same difference applies the other way round: Flat Red is a little lighter, much 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.
See Flat Red on the colour wheel See Vermilion on the colour wheel
All matches: Flat Red equivalents · Vermilion equivalents