You know, weed doesn't stay in the system to long. You'd probably be fine by just drinking tons of water, but if I was in your situation, I'd find a second thing to help out as well, which you have. Drink like 4 litres the day of the test before hand and you'll be fine, and you probably would be even fine if you smoked just a couple day's before with this method. So unless you have a very unhealthy body that doesn't cleanse itself well anymore, you have no need to worry with the combination of plenty of water and this vale solution you've mentioned.