Shame your vet doesn't have a masterject (spring loaded injector), a god send with pigs.
Before that, if they were a restless pig (some of ours you can slowly put the needle through without disturbing them and then attach the syringe), my vet partner would hold the needle in thumb and forefinger, cup it with the other fingers (no syringe) and do a couple of dummy hits on the rump before jabbing on the third go, then if they've not run off you can reattach the syringe. That's still not guaranteed though