The specific act of placing stickers or other small objects on car windows is not in itself illegal. It is an offence, however, under Regulation 32 of the Road Vehicle's (Construction & Use) Regulations 1986 (as amended) to drive a vehicle in which the glass is maintained in such a condition that the vision of the driver is impaired. It is also a requirement of the MOT Test that any windscreen that is damaged or obscured to the extent where the vision of the driver is impaired may fail the test. Basically, this states that in zone A (Zone A is a vertical area 290mm wide, centred on the steering wheel and contained within the swept area of the windscreen - this area is 350mm wide on vehicles over 3.5 tonnes) a windscreen sticker or other obstruction encroaching more than 10mm, and for the swept area, a sticker or other obstruction encroaching more than 40mm would be illegal.
Also don't forget, vinyl should be applied on the outside of the windscreen, as windscreens are laminated on the inside, so will damage the laminate if done inside.