How are you trying to "use more 3.5Gb" of VRAM, or at least how would you go about measuring it? Just trying to see a use-case scenario (compute/rendering etc). Is whatever application you're using set to use more than 3.5GB and/or does it really need to?
As far as I know most 3rd party applications do not work currently. I haven't tried it on mine; I don't use anything other than WattMan via Radeon Settings as they cause more conflicts than solve anything. I'd wait on upcoming updates (whenever that may be) for official RX Vega drivers and utilities.