When mixing liquids by weight I always assumed that a 50/50 VG/PG mixture has a density that lies exactly in between the densities of the components. I faintly remembered from my physical chemistry class that this is usually not the case though. So I found a paper that actually published some data on the real densities of VG (=Glycerol) and PG (=1,2-Propanediol) mixtures:

http://pubs.acs.org/doi/pdf/10.1021/je060496l

For your convenience, I copied the relevant table from the paper:

At 25 °C (298.15 K):

x_vg [mole fraction] | density [g*cm^-3 ] | Molar excess Volume [cm^3 *mol^-1 ] ---|---|--- 0.0000 | 1.0325 | 0.000 0.0841 | 1.0526 | -0.081 0.1712 | 1.0727 | -0.115 0.2615 | 1.0941 | -0.183 0.3552 | 1.1158 | -0.215 0.4524 | 1.1383 | -0.240 0.5534 | 1.1610 | -0.224 0.6585 | 1.1844 | -0.193 0.7677 | 1.2078 | -0.106 0.8815 | 1.2333 | -0.081 1.0000 | 1.2589 | 0.000

Molar weights from the paper:

VG: 92.09 g/mol
PG: 76.10 g/mol

So with this data, you can, for example, calculate:
A solution of
12 mL VG = 15.107 g = 16.40 mmol and
10 mL PG = 10.325 g = 13.56 mmol
giving a mole fraction for VG of 0.5474 (approx. 0.5534)
will result in a density of approx 1.1610 g/mL
and a volume of 21.91 mL
as opposed to the "naive" calculation:
(12/22) * 1.03 + (10/22) * 1.26 = 1.1345 g/mL

These differences are minor and most likely won't have a big influence on your liquids, but I still wanted to get that data out there for reference.