You don't need to use a "forever" button --that's just one way to get something to happen repeatedly in NetLogo. If you're feeling confused by the whole SETUP / GO button thing, then just write all of your video-creating code inside just ONE procedure, like this:
to make-video
clear-all
vid:start-recorder
cro 5
repeat 100 [
ask turtles [
forward 0.1
]
vid:record-view
]
vid:save-recording user-new-file
end