Optimality theory suggests that vegetation optimally adapts to its environment to make best possible use of the available resources to ensure its own persistence. If correct, the theory could be used to make predictions about long-term response of vegetation to environmental change and the resulting CO2 uptake and water use by vegetation. The Vegetation Optimaliy Model (VOM) implement optimality theory under the premise that maximising the vegetation's Net Carbon Profit (NCP) amounts to maximising its capability to persist in a given environment. The NCP is the carbon taken up by photosynthesis minus all the carbon invested into maintenance of the structures required for photosynthesis in the broad sense, including foliage, roots and water transport and storage tissues. Here we present some of the capabilities and shortcomings of the VOM, with a particular focus on the water transport tissues and their costs and benefits in relation to the Net Carbon Profit.