In reply to tony:
I've got one of the original Salomon vests which just came with a bladder, but have now got one of their newer vests with the front soft flasks - the soft flasks are so much better :- you can tell how much you have left, can fill them up from burns, and when half full the water doesn't slosh around like it does when using the front rigid bottles.
I think UD, Inov-8 and various aothers, along with Salomon, are now selling "soft bottles/flasks" so plenty choice out there.
In the Salomon range there are various options depending how much additional carrying capacity you want from 1 litre to 12 litres