Hi all,
I would suggest the better method of reading from the VH400 sensors would be to use 1-wire with a DS2438. We use these to measure voltages from pressure sensors over links of more than 500m and in far worse conditions than you are every going to find in a garden. Lightning has never been a problem except when we stupidly had an earth point 200m from the 1-wire hub on one occasion.
We use standard 4 core underground (direct bury) phone cabling along with gell caps for connections. We also use send 12V down the spare wire and have a local 5V ultra low noise voltage regulator at each sensor, but that is because the pressure sensors absolutely require a 4.72-5.25V supply. The VH400 is very forgiving of supply voltage from the specs on the site.
DS2438 boards are also much cheaper than an arduino, you can address as many as you like and they will survive just about anything.
Andrew