l need front brake calipers and pads,new plastics and a new ignition barrel with keys.could l put car calipers on?could l put other scooter plastics on?

plastics arnt really interchangable.unless you use a couple of hundred cable ties.as for the other bits,best bet is to search on ebay.the bits u need will turn up sooner or later.

No. Front caliper on a Vivacity is tiny! No car caliper will fit and because the Vivacity has a single piston, sliding caliper not even other Peugeot calipers will do.
Ignition barrel has to be bought as lock set with seat lock plus retaining clips and bolts which will cost quite a bit.
Same story with the plastics.

Throw it away, start again.