I use a 20mm stirbar the most. I just got a couple 25mm bars and they what looks to me to mix the same as the 20's. I also have a couple bars with a rib in the middle and it has to be used in a beaker that has a perfectly flat bottom. The jars all have some sort of concave on the bottom that throws the ribbed bar off kilter. I have a 10mm bar that I can mix in 10-20ml jugs. a 15mm bar that mixes 30-50 in the jug but the 20 and now the 25 are my mixing favs for my 100ml mixes.
I don't get a vortex unless I really heat up the VG and once it begins to cool the vortex disappears but it still mixes really well and I get what I think are great juices as a result.
If I do leave the room while mixing I cut a round piece of shelf liner to hold the jar in place incase the stirbar gets thrown, over-spins the fan and vibrates the jar off the mixer.. done that already, 100mls of juice on the floor qualifies to shed a tear.

I should add my juices are 60vg/40pg with 12 nic