If you want to do it manually or for different kinds of plots, you can use the + syntax like so
using VegaLite
x = rand(10)
y1 = rand(10)
y2 = rand(10)
p = @vlplot(width = 500, height = 500) + @vlplot(:line, x = x, y = y1) + @vlplot(:point, x = x, y= y2)