The squier j mascis also seems to be heavier for some reason. It is probably around 9lbs or so on avg. The VMJMs seem to be pretty light, maybe in high 7s on average. I have seen people who have both descibe the mascis as heavier, some describe the Mascis they played as a heavy guitar. While people generally describe the VMJMs as a reasonable weight or even very light.
The general consensus is the Mascis has more consistent quality control. The VMs seem to have more problems with poor finishes, poor neck pockets, pickguard issues, and by the nature of its bridge and trem position is more difficult to setup.
This is just the general trends I have seen. Some Squier Mascis may be pretty light and some VMJMs certainly have very good QC.
Mascis also has a fatter neck, while the VMs are medium in size like a MIM standard Strat.