I do the same syringe into a graduated measuring kitchen thing which is sat on scales,and once my concoction is all in i whisk the crap out of it with a mini mixer,does a cracking job.It then gets poured into the bottle.
Scales. Drip your concentrates into the finished bottle. Pour PG/VG/Nic from fine nozzled bottles straight into the finished bottle. Leave a little headroom for shaking, no need for any syringes, no washing up.
Both. Scales, no mess and quicker, straight into your mix bottle, i sometimes use a syringe for the hard to squeeze bottles i.e the flavour concentrates (i'm too lazy to put 15-20% in by drops) and your vg/pg/nic will probably need to be put into squeezy bottles with nozzles.
Also mixing by weight with scales is a lot more accurate than measuring with cylinders, syringes etc. This tends to be more important with multiple flavoured recipes though when you are dealing with smaller percentages.