It sounds like the boot isn't compatible with your foot.
Although the vento 6 liner is mouldable, heat moulding liners won't do shit if you are trying to create heaps of volume in the boot, and the money you spend on shellwork trying to make them fit well, will probably outweigh the cost of the boot.
If for some reason the ONLY issue is over the instep, then it is possible for a tech to grind down the bootboard under the liner which will drop your foot down in the boot a little, and should alleviate some pressure over the top of your foot.
Then again, grinding down the bootboard will reposition your foot slightly, so it could even create new problems.
As said above, boots are the most important piece of ski equipment you will own, and given how vastly different peoples feet can be, it's a great idea to get them professionally fitted, even if they cost you more than your skis.
Assuming your feet have stopped growing, they will last you many years and will outlast the rest of your gear.
Have a read of this: