if you have a return line that cycles veggie back to the tank, and you operate 100% of the time in a warm enviroment when you want to use veggie you might get away with not actually heating the tank. I've been contemplating converting my vw over to veggie and not having engine coolant lines submerged in the tank.
You would have to do a test, and figure out how fast fuel flows through your fuel lines when the engine is at max rpm. I estimate that for every gallon of fuel a diesel engine burns, it returns 10 gallons to the tank, so there is a lot of fuel cycling going on. If your using your plate heat exchanger that looks like this:
you should do an experiment: figure out what the flow rate is for your fuel. using the heat exchanger with your engine up to temperature, measure the temperature of the wvo before/after it goes through the heat exchanger. With 1 or 2 heat exchangers you may be able to heat your fuel up enough that you don't need to actually heat the fuel in the tank.....although the fuel in the tank will heat up after a few hours of driving because you're returning hot wvo to the tank all the time.
another thing you can use in conjunction to other heaters are 110 volt "band heaters" they only take like 100 watt or something, and are designed to heat liquid inside of a pipe. They look like this : These can be handy to have on the wvo line just before it enters the injector pump.