Ditto- If something is sold as 99+% Vg then it really should not vary in weight by any amount we could measure regardless of the vendor.
The one thing that a person Might encounter in weighing VG or VG Nicotine Base is a Variation due to Water and Nicotine content.
If Pure Vegetable Glycerin weighs 1.261g/ml but it has been Diluted with Water (1g/ml) then the Diluted solution will weigh Less.
How Much Less? That depends on the Amount the Pure is Diluted by. But the weight would be Bound between 1.261g/ml and 1.000g/ml
I dilute my Pure VG with about 5% Water. So the Diluted VG I use to Mix with is about...
((.95 x 1.261) + (.05 x 1.00))g/ml => 1.25g/ml
I just Round it Off to 1.200g/ml because I am concerned with Levels of Accuracy at the +/- .05g/ml scale for VG. And 1.2mg/ml is an Easier Number to do Mental Math if I want to.
The Other Factor, which we might as well consider, is the Weight of the Nicotine.
If a Person is using 100mg/ml Nicotine Base which is 100% VG they Actual are Not Using 100% by Volume. They are Using 90% VG by Volume. And the Remaining 10% is the Nicotine.
And the Nicotine weighs Less than Pure VG.
So 1ml of 100mg/ml 100% VG Nicotine Base should weigh about 1.2376g/ml
Whereas 1ml of say 18mg/ml 100% VG Nicotine Base should weigh about 1.2563/ml
A Very Small Variation of .0187g/ml. But something that a person should see.