In the UK supermarkets sell 'table water' (purified) for 16p (about 25c) for 2 litres. It's so cheap I always use that for purity of taste. It's been a couple of years since I lived in the US but I seem to remember it's pretty cheap there too.
I would leave off any back sweetening for now. I was about to add additives to the Chardonnay I mentioned in my other reply but after two months it was perfect. Bearing in mind the Viognier is designed to finally taste very floral with a touch of honeyed sweetness any extra sweetness you add...
I'm also just at the bottling stage with this one and yes mine also tastes quite flat, no fruit and quite acidic. Which is a bit odd as I got this one for it's 'floral bouquet exotic finish' etc. It currently has no taste at all other than a bit acidic. However, I bottled a Kenridge Classic...