Unless it has changed the Maximum amount of vRAM that BIOS can assign to a APU is 2 GBs. If you need more vRAM you will need to install a separate GPU card with 4 GB or more of vRAM on it.
If in BIOS it shows the maximum of 2 GBs than that is the most you can assign to your APU (Processor with graphics). There is no way to increase it past the maximum limitation.
Im having the same problem, i see some people with the same APU that can reache 16 gb in the BIOS, It means that my motherboard can't reache It?
Depends on your Motherboard BIOS that determines how much of the APU Integrated Graphics uses System RAM Memory as vRAM.
Maybe with the newer AMD APUs it can allot more then 2GBs of System RAM Memory.
You need to check your BIOS Settings to see what is the maximum vRAM using System RAM Memory that can be assigned to your APU.
NOTE: Searching the internet I see that Asus has increased the vRAM of the 3400G to 3GB Maximum in BIOS. According to this Tom's Hardware thread the reason why you can't assign more than a certain amount of vRAM to a APU: