Recently, data.table allows .. prefix to allow reference to parent-scoped variables in j:
library(data.table)
v_names = c('a', 'b')
DT = data.table(a = 1:3, b = 4:6, c = 7:9)
# allows subsetting to 'a' and 'b' dynamically,
# as a work-around to the default 'with'
# behavior of data.table scoping
DT[ , ..v_names]
If this is the complete script, RStudio will throw a warning on the vnames line if the "warn about unused variables" flag is on.