Can you give your Swiss motorway vignette to a friend after using it on your car?

1. No, it is definitely not legal. The sticker is also designed to be difficult to remove cleanly (if you look carefully you will see that once it's on the windshield, it's not a simple square but a kind of spiral with many nooks, not at all like a random sticker or even the Austrian motorway vignette) and invalid if not affixed to the windshield.

2. If you manage to get if off and affix it to another windshield cleanly, they have no way to tell. I know people who have done so and I have heard of various tricks to make removing it easier. But it is more difficult than you think.

The other answer already addressed the fact that it's illegal, let me answer your second question:

The sticker is designed in a way that it will be destroyed if you try to remove it. It has a special perforation and is much harder to remove than ordinary car stickers. (See this picture for instance) It is also illegal to use tricks to make it easier to remove such as stick it to a piece of transparent foil or something like that:

Never attach it with adhesive tape or film so that it is easy to remove and transfer to another vehicle. You could be fined several hundred fines if the police catch you.

Note that the sticker is so tricky to remove, that there's a lot of websites online teaching you some very involved techniques of doing it (the sticker will still get destroyed):

What’s the easiest way to remove old vignettes?

The best way is to use a ceramic hob scraper. It’s even easier if you warm the sticker first using a hairdryer. Any adhesive residues can be cleaned off with nail polish remover.

Of course as one of the other answers says, if you do manage to remove it without destroying it, there'll be no way of noticing it.

900 caught 2012.

Article. 7 Motorway sticker: (i've double checked this translation, i'm a translator)

1 The motorway charge is paid by the purchase of a motorway sticker (sticker).

2 The sticker shall be affixed directly to the vehicle before the use of a national highway subject to the charge.

3 It can only be transmitted with the vehicle.

4 It is no longer valid in the following cases:

a. It is detached from the vehicle after being glued correctly; (notice the semicolon looks like a smiley. perhaps you can glue it with a smidgeon of butter; a little bit way, a little bit woo, you're a geezer)

b. It is detached from its support without being glued directly to the vehicle.

Art. 14 Fines

1 Any person who, contrary to ss. 3 to 5, 7 and 8, intentionally or negligently borrows a national highway subject to the levy at the wheel of a vehicle or uses the vignette in a manner contrary to these provisions shall be liable to a fine of 200 francs.

Use of butter is less of a serious fraud than use of a removable substrate, because the latter is applied forgery, whereas the former is only the use of a bit of butter on the vignette.

you must affix it directly, however, the notion of it being properly glued or not is not discussed in the legal article... The vignette is void if it is removed after having been "PROPERLY STUCK ON" however, they place a semi-colon after that voidance condition; not a full stop; and they do not discuss any scenarios whereby it the vignette is removed after it is not properly glued on.

I will recap: It cannot be transferred between vehicles. It must be stuck directly to the car... It is void if it is removed after having been STUCK CORRECTLY" If it is removed after having being glued incorrectly, using butter, the legal statement is implicit and not explicit, there is room for interpretation.

It's not a crime(delit), it's an infraction of the law(same as speeding), but swiss people with multiple cars and only one vignette find ways to move the vignette from one car to another. there is a 2 month vignette for 40 francs. There are videos about how to unstick them here https://www.youtube.com/watch?v=hviCgzS5eAY

the fine may have changed it may be 500fr.

One unfortunate guy was condemned for "falsifiction of government certificates of value" for carefully applying the sticker to another support and moving it: https://www.letemps.ch/suisse/2015/07/23/on-ne-triche-collage-vignette-autoroutiere

in practice this kind of fraud is notoriously common. one guy writes " i apply a thin layer of greasy cream before applying the sticker, so it is very easy to remove, is it legal ? " they put butter too. they don't even know very well themselves if the trick is legal. it's better to do that than carefully applying it to a second sticker and cutting around it.

The number of fraudsters caught every year is in the 100ds..: http://www.tdg.ch/suisse/Les-fraudeurs-rivalisent-d-imagination-pour-la-vignette-/story/16495591