It mostly depended on the manufacturer.
Atari, Midway, and Universal screen printed theirs.
Williams stencilled theirs (Defender, Joust, etc).
Sega/Gremlin used decals. After converting games became the thing to do, most everything went to a roughly 18 x 24 decal. That's also why you'll see so many posts about stripping off black paint on old cabinets. Paint it black, slap on a sticker, and you have a new cabinet despite the fact that Peter Pepper's hat makes a funny shape on top. As far as I know, there weren't any manufacturers that used cut vinyl like you get from a sign shop. Many of the decals are screen-printed vinyl though.