I don't know too much about vacuum forming... but do you know what the maximum thickness of plastic you can use to make parts out of?
With a simple easy setup like in the video, you can form plastic up to about a quarter of an inch thick. I've made parts the size of the one in the video that my wife and I can both stand on without breaking them. (Of course the rigidity & strength depend on the shape.)
When you form thick plastic as shown in the video, without a high vacuum system, you don't get very sharp detail---the shape gets smoothed out by the thickness of the plastic, and somewhat more.
You can get better detail with a high vacuum system, which pulls several times harder than a vacuum cleaner. (You can use a converted bike pump, or electric tire inflator, or kitchen food sealer, or a "nebulizer" pump from a thrift store.) For maximum detail, you want to suck the plastic into a concave mold, rather than draping it over a positive mold as in the video. (The side of the plastic that's against the mold has the best detail; the other side is smoothed out more.)
I'd think that for most robot purposes, plain vacuum forming of 1/8" plastic over a positive mold with a vacuum cleaner would be fine---you can make smooth compound curves with curve radii of about 3/16" for convex stuff, or 3/8" for concave areas. (Somewhat finer detail does show through, but in muted form.)
People seem to like rounded-off robot stuff, and the parts are generally convex, so a cheap easy vacuum forming setup can be very useful for making your bot look professionally made.
With a fancier setup, you can form plastic up to 3/8" or 1/2" thick, but I think that's overkill for most robot purposes, and I haven't tried it yet.
