Yes, connecting it to your NMEA 2K network will allow you to get fuel consumption on your Lowrance. You should also be able to send the speed data to your Vesselview and configure it to show speed and MPG, although the manual is a little unclear. From page 29 of the VV403 owner's manual:
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Fuel
The VesselView Fuel screen displays the total fuel based on the current tank data entered in the tank configuration through the tanks menu or the setup wizard. The lower portion of the screen will show the fuel flow, or fuel consumption in volume per hour. The volume will be dependent on the units of measure that was selected during the setup wizard process. Units of measure can be changed at any time by navigating to Menu>Settings>Preferences>Units
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So not 100% clear if you can show MPG on your VV403, but worthing giving a try. Regardless, your Lowrance will be able to display that info.
Note that there are two ways for the VV to show your remaining fuel: 1) based on the reading from the sender in the fuel tank, or 2) based on fuel consumed as calculated by the engines CPU. There are pros and cons to each. Both will require you to enter your fuel tank size. 1) requires you to calibrate the tank, which involves starting with an empty tank and entering into the VV when as you fill up at 0%, 25%, 50%, 75% and 100%. It will still be an estimate though and things like how your boat is sitting will affect the reading. The advantage is that once you set it up you don't have to do anything else.
Option 2 will give you a more accurate reading for how much fuel is in your tank, but will require you enter into the system whenever you add fuel to your tank, but that is pretty straighforward. I use this method, partly because I didn't have the ability to completely drain both fuel tanks when I repowered in order to calibrate for Option 1, but mainly because I prefer having a more accurate readout of my remaining fuel.
