ΔE 19.6, loose: different enough that you should pick on purpose. Viridian Green is a little lighter, much more saturated, cooler than Medieval Forest.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Viridian Green is a little lighter, much more saturated, cooler than Medieval Forest.
The same difference applies the other way round: Medieval Forest is a little darker, much more muted, warmer.
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 Medieval Forest on the colour wheel See Viridian Green on the colour wheel
All matches: Medieval Forest equivalents · Viridian Green equivalents