Veg is thicker than pump diesel so a couple of things can happen, even in the 'summer' time.
As it's thicker, the pump is sucking harder and that means if there is any weakness in the fuel lines, there's a higher chance of it sucking in air which will always make the engine run rough. Also as it's thicker, it doesn't pass through the fuel filter or the mini filter so well, so if either of these is even a bit congested, the engine will suffer fuel starvation.
Adding 15-20% petrol to veg will bring the viscosity back to that of pump diesel, but will then have the effect of lowering the cetane level a bit. Adding a cetane enhancer such as vegiboost will bring that back to 'normal'. Petrol also stops the darn stuff turning into margarine in the winter time.
As long as you get the veg cheap enough, then it does work out cheaper than pump diesel and definitely if you can get WVO. However, with biodiesel, the viscosity issues go and the risk of having veg oil gumming the works also goes. Note to self, must finish building my bio processor!