I use 100ml beakers and make 60ml batches.[ with only a few exceptions]
If I make a 30ml batch , in the 100ml beaker, I just use a smaller stir bar.
There are different size magnetic stirrers....size meaning the strength of the magnets themselves,
and also the rpm's......magnets also lose potency [strength] over time....causes are heat, electrical, other magnets, etc
If you purchase a used lab mag mixer, chances are it is at half strength when you get it....and one reason they changed them out at the lab.
If buying a new mag mixer, check the capacity it says it will stir, and pick one at least double what you plan to mix, especially if you mix heavy on the VG. A good mag mixer under $200 will last you a lifetime for personal use. I have even seen some SH-2's for about $125...new.
You can also start a larger batch with a small stir bar, then change it to a larger one after it mixes a while.....small stir bars are not just for small batches.
Mixing 500ml of VG in a lab setting is a small batch when using the proper magnets. I have mixed 1000ml+ of regular motor oil at room temp with no problems......over and over.